MB-11706 Fix the race condition during exit

There can be a condition where before sending the exit to
process (cleaner, compactor, updater), process has exited and
that exit message has not yet processed.So when we try to send
exit message via port_command, it gives us badarg since the
process has already exited. Ignored the checking for the status,
since process may have exited.

Change-Id: Ia210b87363f3057e7d579f220b28428349b726cc
Reviewed-on: http://review.couchbase.org/39348
Reviewed-by: Nimish Gupta <nimish@couchbase.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: buildbot <build@couchbase.com>