mon: MonOpRequest: send_reply() belongs in the Monitor class

mon: MonOpRequest: send_reply() belongs in the Monitor class

Op Requests should have no business replying to messages. Besides,
given the Monitor is currently the place to do this, because it is the
one with access to all things that may be required to validate state
(e.g., quorum features), permanently moving this code to the Monitor
class also avoid having duplicate/very similar code in two distinct
places.

Signed-off-by: Joao Eduardo Luis <joao@suse.de>