mon/OSDMonitor: track history and past_intervals for creating pgs

mon/OSDMonitor: track history and past_intervals for creating pgs

PG create messages from mons are the last case where the OSD may have to
scan an unbounded number of old maps in order to construct a valid
pg_history_t and PastIntervals.  Try to avoid making that a difficult
case by maintaining those structures on the monitor.

It is still possible that the mon may send a pg create message to the OSD
and it sits in a message queue for a very long time, but this would be a
very difficult situation to get into, and is no different from inter-OSD
messages that include history and past_intervals.

Signed-off-by: Sage Weil <sage@redhat.com>