mon: PGMonitor: assert on update_from_paxos() if incremental is empty

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
mon: Monitor: keep track of paxos' sync version

This avoids uncommitted versions to be taken into consideration after
a sync has finished.

Fixes: #4026
Fixes: #4037
Fixes: #4040

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>