mon: accumulate a single pending transaction and propose it all at once

mon: accumulate a single pending transaction and propose it all at once

Previous we would queue lots of distinct encoded Transactions from various
callers, usually one per PaxosService.  These would be sent through paxos
one at a time.

If there is a completed transaction there is no reason to delay; it is
more efficient to push it through immediately.  Since we will propose
anything pending right when we finish, there is minimal opportunity for
other work to get done.

Instead, accumulate everything in a single MonitorDBStore::Transaction and
propose all pending changes all at once.  Encode at propose time and
expose the Transaction to the callers so they can add their changes.

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