common/config: hold lock while accessing mutable container

common/config: hold lock while accessing mutable container

The 'call_gate_leave' method was accessing the 'obs_call_gate' map
without holding the required lock. This data structure could be
manipulated by another thread underneath the observer callback
thread context.

Fixes: https://tracker.ceph.com/issues/41354
Signed-off-by: Jason Dillaman <dillaman@redhat.com>
(cherry picked from commit 0cc2dd7bd1021dd8b5e22850495ce97da7d98716)

Conflicts:
	src/common/config_proxy.h: logic incorporated into config.cc