MDSMonitor: cleanup and protect fsmap access

MDSMonitor: cleanup and protect fsmap access

This commit is designed to permanently protect the current epoch FSMap from
accidental changes. Modifications to the pending_fsmap are protected by
checking if the monitor is the leader. Additionally, code must explicitly
request a writeable reference.

Fixes http://tracker.ceph.com/issues/23762

Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>
(cherry picked from commit 624efc64323f99b2e843f376879c1080276e036f)

Conflicts:
	src/mon/MDSMonitor.cc
	src/mon/MDSMonitor.h