mon/MonClient: remove stray _finish_hunting() calls

Callig _finish_hunting() clears out the bool hunting flag, which means we 
don't retry by connection to another mon periodically.  Instead, we send 
keepalives every 10s.  But, since we aren't yet in state HAVE_SESSION, we 
don't check that the keepalives are getting responses.  This means that an 
ill-timed connection reset (say, after we get a MonMap, but before we finish
authenticating) can drop the monc into a black hole that does not retry.

Instead, we should *only* call _finish_hunting() when we complete the 
authentication handshake.

Fixes: #8278 Backport: firefly, dumpling Signed-off-by: Sage Weil
<sage@inktank.com>