mon/PaxosService: add post_paxos_update() hook

Some services need to update internal state based on other service's
state, and thus need to be run after everyone has pulled their info out of
paxos.

Backport: cuttlefish
Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit 131686980f0a930d5de7cbce8234fead5bd438b6)
mon/PGMonitor: use post_paxos_update, not init, to refresh from osdmap

We do two things here:
 - make init an one-time unconditional init method, which is what the
   health service expects/needs.
 - switch PGMonitor::init to be post_paxos_update() which is called after
   the other services update, which is what PGMonitor really needs.

This is a new version of the fix originally in commit
a2fe0137946541e7b3b537698e1865fbce974ca6 (and those around it).  That is,
this re-fixes a problem where osds do not see pg creates from their
subscribe due to map_pg_creates() not getting called.

Backport: cuttlefish
Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit e635c47851d185eda557e36bdc4bf3775f7b87a2)

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