mon/Paxos: drop unnecessary last_committed loads

Drop (apparently) ad-hoc refreshes of last_committed from the store.
These are unnecessary and confusing.

Signed-off-by: Sage Weil <sage@inktank.com>
mon/Paxos: assert that the store gives us back what we just wrote

In bug #5424 I observed leveldb failing internally and then returning
bad info.  We then hit a random/confusing assert.  Try to detect this
earlier by verifying that a get of a just-written last_committed gives
us back the right thing.

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