Re-use the accept mutex on a graceful restart

Previously the clean-up of the pconf pool during a graceful
restart in the main process would destroy the mutex.  For the
sysv and pthread mutex types, the fact that the parent process
destroyed the mutex caused operations on it in the child process
to fail immediately, defeating the purpose of GracefulExitDelay.
This was not a problem for the fcntl mutex type (because the
destroy function merely closes the underlying file), but the
fcntl mutex type has been found to be spinlock-heavy.

With this commit the accept mutex is tied to the global pool
instead of the pconf pool and worker_run reuses the mutex
after it has been created once.

As a consequence of that, to change the accept mutex type
a full restart is needed.
Merge pull request #2 from fastly/jozef/reuse-accept-mutex

Re-use the accept mutex on a graceful restart