mon: ConfigKeyService: stash config keys on the monitor

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.

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I went with the name Greg suggested on the last review, and made the class reflect just that.

If we happen to extend the monitor to support other commands relying on the same approach -- i.e., storing stuff on the monitor's store, accessible to the user --, we can always create a GenericKVStore class responsible for handling the different stores/services, among which we would find the ConfigKeyService, much like what was done for the HealthMonitor and the DataHealthService.