cmogstored 1.2.1 - fix graceful shutdown failure

This release only fixes an assertion failure during graceful shutdown
while MogileFS fsck is running with checksumming enabled.

This only affects users running fsck with checksumming enabled during a
graceful shutdown of cmogstored.  For upgrading cmogstored it is
recommended to:

1) stop fsck on the trackers (via "mogadm fsck stop")
2) wait for all tracker queues to drain and stop sending
   fsck traffic to the affected host.  You may wish to
   "!want 0 fsck" on all your trackers and wait for the
   fsck workers to stop.
3) upgrade cmogstored (in place upgrade works)

There are also several code comment updates for internal
components of cmogstored which may interest potential hackers.