mon: AuthMonitor: delete auth_handler while increasing max_global_id

By not deleting and setting NULL the session's auth_handler, we could
hit a scenario in which we'd end up dispatching a previously-wait-listed
auth message and we wouldn't start its auth session.

This only happened when increasing max_global_id via Paxos (in which case
we would wait-list the message) and would only be noticeable when running
with cephx disabled.

Fixes: #4519

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>