mon: add a cache layer over MonitorDBStore

mon: add a cache layer over MonitorDBStore

the cache of of leveldb does not perform well under some condition,
so we need a cache in our own stack.

* add an option "mon_osd_cache_size" to control the size of cache size
  of MonitorDBStore.

Fixes: #12638
Signed-off-by: Kefu Chai <kchai@redhat.com>
(cherry picked from commit 5e99a578050976ca22b549812ac80d494fe7041d)

Conflicts:
	src/mon/OSDMonitor.h
minor differences in OSDMonitor ctor