mon: always set up session; move waitlist logic

mon: always set up session; move waitlist logic

Unconditionally attach a Session, so that when we push an op back through
dispatch_op it isn't in a weird broken state.

Move the waitinglist check to the bottom.  Use s->global_id == 0 to tell
whether a client has authenticated, and zap sessions based on that.

Signed-off-by: Sage Weil <sage@redhat.com>