mon/PGMonitor: reset in-core PGMap if on-disk format changes

We might have a sequence like:

- start mon, load pgmap 100
- sync
 - including a format upgrade at say v 150
- refresh
 - see format_version==1, and try read pgmap:101 as new format

This simply clears our in-memory state if we see that the format has
changed.  That will make update_from_paxos reload the latest and prevent
it from walking through the old and useless inc updates.

Note: this does not affect the auth monitor because we unconditionally
load the latest map in update_from_paxos on upgrade.  Also, the upgrade
there wasn't a format change--just a translation of cap strings from the
old to new style.

Fixes: #5764
Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Greg Farnum <greg@inktank.com>