mon: move required_features back into Monitor

This is simpler and cleaner.

Signed-off-by: Sage Weil <sage@inktank.com>
mon: tell peers missing features during probe

Use a new probe op to inform mons that they are missing features during
the earliest probe phase.  This prevents them from getting as far as
the sync entirely if they are too old.

We still need to refuse to speak to them if they try to call an election,
which they could do based on their replies from other peers.

Note that old clients will assert on getting a message type string they
don't understand, so we need to be careful not to send the probe reply
to older clients.  The feature bit we use is not precise in that it does
not cover recent dev releases, but it does work for dumpling and emperor.

Signed-off-by: Sage Weil <sage@inktank.com>