mon: allow subscription to kv/config-key data

mon: allow subscription to kv/config-key data

Allow subscription to config-key/kv data.  Initially we'll send a full
dump of the prefix.  As changes occur, we'll send incremental diffs,
unless the subscriber is too far behind, in which case we'll send a full
dump again.

There is a new message, MKVData, to support this.

No compat issues since old clients won't subscribe to this stream unless
they know how to handle it.

Signed-off-by: Sage Weil <sage@newdream.net>