mon: include any new paxos commits in each sync CHUNK message

We already take note of the paxos version when we begin the sync.  As
sync progresses and there are new paxos commits/txns, include those
and update last_committed, so that when sync completes we will have
a full view of everything that happened during sync.

Note that this does not introduce any compatibility change:

 old provider, new requester:
  - msg->version will no be set except for last msg; no change
 new provider, old requester:
  - provider will send the keys along
  - the requester will not update last_committed until the very end
  - requester will still do a final sync of paxos prefix

Signed-off-by: Sage Weil <sage@inktank.com>
mon/Paxos: configure minimum paxos txns separately

We were using paxos_max_join_drift to control the minimum number of
paxos transactions to keep around.  Instead, make this explicit, and
separate from the join drift.

Signed-off-by: Sage Weil <sage@inktank.com>
mon/Paxos: increase paxos max join drift

A value of 10 is too aggressive for large, long-running syncs. 100 is
about 2 minutes of activity at most, which should be a more forgiving
buffer.

Signed-off-by: Sage Weil <sage@inktank.com>
mon: uninline _trim_enable so we can debug it

Signed-off-by: Sage Weil <sage@inktank.com>
mon/Paxos: log trim enable/disable

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