mon: discover mon addrs, names during election state too

Currently we only detect new mon addrs and names during the probing phase. 
For non-trivial clusters, this means we can get into a sticky spot when we
discover enough peers to form an quorum, but not all of them, and the 
undiscovered ones are enough to break the mon ranks and prevent an election.

One way to work around this is to continue addr and name discovery during the
election.  We should also consider making the ranks less sensitive to the
undefined addrs; that is a separate change.

Fixes: #4924 Backport: dumpling Signed-off-by: Sage Weil <sage@inktank.com> 
Tested-by: Bernhard Glomm <bernhard.glomm@ecologic.eu>