Elector: ignore messages from mons without required feature capabilities

We maintain a list of required_features which the other monitor's features
must supply. This starts out at 0 and is initialized from the monitor's
list of features whenever we start electing.
Despite the scary sound of "just ignore it", this is safe: the monitor
will only record features as required once a quorum has formed in which
every monitor supports them. After that happens, monitors which do not
support those features will be unable to read the whole mon store/understand
the pg reports/whatever else, so letting them into the quorum would be buggy
behavior.
So if we ignore a monitor, it will not be able to start nor join
an election round with anybody who was in our quorum -- that is, the
ignored monitor cannot form a separate quorum. By ignoring it here, we
also prevent it from endlessly calling elections against the real
quorum.

Unfortunately there is no way to communicate to old monitors that they
cannot join the quorum -- there are no existing messages for that purpose,
and eg adding a new op to the MMonElection message will just cause it
to crash, which we don't want to do either.

Fixes: #7215

Signed-off-by: Greg Farnum <greg@inktank.com>
Elector: send an OP_NAK MMonElection to old peers who support it

Only new monitors support receiving OP_NAK from a peer without crashing, but
when we add new required features in the future, our monitors can accept
an OP_NAK message which tells them what features they're missing. Then they
will print out an error message and shut down cleanly.

Signed-off-by: Greg Farnum <greg@inktank.com>