mon: MonitorDBStore: add store iterators to obtain chunks for sync

mon: MonitorDBStore: add store iterators to obtain chunks for sync

We created an interface specific to the MonitorDBStore, which can be used
to create iterators to obtain chunks for sync.

Two different iterators were defined: one that will iterate over the whole
store, focusing on the specified set of prefixes; another that will
iterate over only one specific prefix.

These two different iterators allow us build the sync process in two
distinct phases: 1) obtain all key/value pairs for paxos and all paxos
services, bundle them in chunks and send them over the wire; and 2) obtain
all the paxos versions, bundle them in chunks and send them over the wire.

Also, we are currently considering a chunk to be (at most) 1 MB worth of
data, although it can be tuned using 'mon_sync_max_payload_size' option.

mon: MonitorDBStore: add crc support when --mon-sync-debug is set

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