unicorn 4.8.0 - big internal changes, but compatible

This release contains fairly major internal workings of master-to-worker
notifications.  The master process no longer sends signals to workers
for most tasks.  This works around some compatibility issues with some
versions of the "pg" gem (and potentially any other code which may not
handle EINTR properly).  One extra benefit is it also helps stray
workers notice a rare, unexpected master death more easily.  Workers
continue to (and will always) accept existing signals for compatibility
with tools/scripts which may signal workers.

PID file are always written early (even on upgrade) again to avoid
breaking strange monitoring setups which use PID files.  Keep in mind we
have always discouraged monitoring based on PID files as they are
fragile.

We now avoid bubbling IOError to the Rack app on premature client
disconnects when streaming the input body.  This is usually not a
problem with nginx, but may be on some LAN setups without nginx).

Thanks to Sam Saffron, Jimmy Soho, Rodrigo Rosenfeld Rosas,
Michael Fischer, and Andrew Hobson for their help with this release.

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