mon: MonitorDBStore: add 'get_iterator()' method to iterate over a prefix

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
mon: ConfigKeyService: stash config keys on the monitor

Building up on the Single-Paxos and our existing k/v store that backs
the monitor, we now introduce a simple service so that the monitors
act as a generic k/v store available to the cluster, in which a user
can stash (and later obtain) configuration keys at his own discretion.

Users can put, get, delete, list and check for values using the
following commands:

 - ceph config-key put <key> <value>
  or
 - ceph config-key put <key> -i <in-file>

 - ceph config-key get <key>
  or
 - ceph config-key get <key> -o <out-file>

 - ceph config-key delete <key>

 - ceph config-key list

 - ceph config-key exists <key>

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