mon: elector: return after triggering a new election

mon: elector: return after triggering a new election

When receiving an old propose, we were correctly triggering a new election
but not then returning out of receive_propose(), so we processed the
"should I defer" logic and perhaps sent out a deferal (in the current epoch!).

Fixes: https://tracker.ceph.com/issues/42977
Signed-off-by: Greg Farnum <gfarnum@redhat.com>
(cherry picked from commit 337d12ba802d5b31db36be25c3349b9b67ec17ba)

Conflicts:
	src/mon/ElectionLogic.cc
- file does not exist in nautilus: made the change manually in
  src/mon/Elector.cc