cmogstored 1.3.0 - many improvements

There are no changes from 1.3.0rc2.

For the most part, cmogstored 1.2.2 works well, but 1.3 contains some
fairly major changes and improvements.

cmogstored CPU usage may be higher than other servers because it's
designed to use whatever resources it has at its disposal to
distribute load to different storage devices.  cmogstored 1.3
continues this, but it should be safer to lower thread counts
without hurting performance too much for non-dedicated servers.

cmogstored 1.3 contains improvements for storage hosts at the
extremes ends of the performance scale.  For large machines with many
cores, memory/thread usage is reduced because we had too many acceptor
threads.  There are more improvements for smaller machines, especially
those with slow/imbalanced drive speeds and few CPUs.  Some of the
improvements came from my testing with ancient single-core machines,
others came from testing on 24-core machines :)

Major features in 1.3:

ioq - a I/O queues for all MogileFS requests
--------------------------------------------

The new I/O queue (ioq) implements the equivalent of AIO channels
functionality from Perlbal/mogstored.  This feature prevents a
failing/overloaded disk from monopolizing all the threads in the system.

Since cmogstored uses threads directly (and not AIO), the common
(uncontended case) behaves like a successful sem_wait with POSIX
semaphores.  Queueing+rescheduling only occurs in the contended case
(unlike with AIO-style APIs, where request are always queued).  I
experimented with, but did not use POSIX semaphores as contention would
still starve the thread pool.

Unlike the old fsck_queue, ioq is based on the MogileFS devid in the URL
and not the st_dev ID of the actual underlying file.  This is less
correct from a systems perspective, but should make no difference for
normal production deployments (which are expected to use one MogileFS
devid for each st_dev ID) and has several advantages:

1) testing/mock deploys of this feature with mock deploys is easier

2) we do not require any additional filesystem syscall (open/*stat)
   to look up the ioq based on st_dev, so we can use ioq to avoid
   stalls from slow open/openat/stat/fstatat/unlink/unlinkat syscalls.

Otherwise, the implementation of this very closely resembles the old
fsck queue implementation, but is generic across HTTP and sidechannel
clients.  The existing fsck queue functionality is now implemented using
ioq.  Thus, fsck queue functionality is mapped by the MogileFS devid and
not the system st_dev ID as a result of this change.

One benefit of this feature is the ability to run fewer aio_threads
safely without worrying about cross-device contention on machines with
limited resources or few disks (or not solely dedicated to MogileFS
storage).

The capacity of these I/O queues is automatically scaled to the number
of available aio_threads, so they can change dynamically while your
admin is tuning "SERVER aio_threads = XX"

However, on a dedicated storage node, running many aio_threads (as is
the default) should still be beneficial.  Having more threads can keep
the internal I/O queues of the kernel and storage hardware more
populated and can improve throughput.

thread shutdown fixes (epoll)
-----------------------------

Our previous reliance on pthreads cancellation primitives left us open
to a small race condition where I/O events (from epoll) could be lost
during graceful shutdown or thread reduction via
"SERVER aio_threads = XX".  We no longer rely on pthreads cancellation
for stopping threads and instead implement explicit check points for
epoll.

This did not affect kqueue users, but the code is simpler and more
consistent across epoll/kqueue implementations.

Graceful shutdown improvements
------------------------------

The addition of our I/O queueing and use of our custom thread shutdown
API also allowed us to improve the responsiveness and fairness when the
process enters graceful shutdown mode.  This improves fairness and
avoids client-side timeouts when large PUT requests are being issued
over a fast network to slow disks during graceful shutdown.

Currently, graceful shutdown remains single-threaded, but we will likely
become multi-threaded in the future (like normal runtime).

Miscellaneous fixes and improvements
------------------------------------

Further improved matching for (Linux) device-mapper setups where the
same device (not symlinks) appears multiple times in /dev

aio_threads count is automatically updated when new devices are
added/removed.  This is currently synced to MOG_DISK_USAGE_INTERVAL, but
will use inotify (or the kqueue equivalent) in the future.

HTTP read buffers grow monotonically (up to 64K) and always use aligned
memory.  This allows deployments which pass large HTTP headers do not
trigger unnecessary reallocations.  Deployments which use small HTTP
headers should notice no memory increase.

Acceptor threads are now limited to two per process instead of being
scaled to CPU count.  This avoids excessive threads/memory usage and
contention of kernel-level mutexes for large multi-core machines.

The gnulib version used for building the tarball is now included in the
tarball for ease-of-reproducibility.

Additional tests for uncommon error conditions using the fault-injection
capabilities of GNU ld.

The "shutdown" command over the sidechannel is more responsive for epoll
users.

Improved reporting of failed requests during PUT requests.  Again, I run
MogileFS instances on some of the most horrible networks on the planet[2]

fix LIB_CLOCK_GETTIME linkage on some toolchains.

"SERVER mogstored.persist_client = (0|1)" over the sidechannel is supported
for compatibility with Perlbal/mogstored

The Status: header is no longer returned on HTTP responses.  All known
MogileFS clients parse the HTTP status response correctly without the
need for the Status: header.  Neither Perlbal nor nginx set the Status:
header on responses, so this is unlikely to introduce incompatibilities.
The Status: header was originally inherited from HTTP servers which had
to deal with a much larger range of (non-compliant) clients.