Elector: send an OP_NAK MMonElection to old peers who support it

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.
(Unfortunately, doing a clean shutdown from here would require a lot of
infrastructure, so we just call exit(0).)

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