mon/PGMonitor: store PGMap directly in store, bypassing PaxosService stash_full

mon/PGMonitor: store PGMap directly in store, bypassing PaxosService stash_full

Instead of encoding incrementals and periodically dumping the whole encoded
PGMap, instead store everything in a range of keys, and update them
between versions using transactions.  The per-version values are now
breadcrumbs indicating which keys were dirtied so they can be refreshed
via update_from_paxos().

This has several benefits:
 - we avoid every encoding the entire PGMap
 - we avoid dumping that blob into leveldb keys
 - we limit the amount of data living in forward-moving keys, which leveldb
   has a hard time compacting away
 - pgmap data instead lives over a fixed range of keys, which leveldb
   excels at
 - we only keep the latest copy of the PGMap (which is all we care about)

Bump the internal monitor protocol version.

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