mon: Monitor: drop connectionless messages for non-mon on dispatch

If we are a client sending a message to a monitor, we are expected to
establish a connection first.  Monitors are the exception.

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
mon: Monitor: drop client msg if no session exists and msg is not MAuth

If we are not a monitor and we don't have a session yet, we must first
authenticate with the cluster.  Therefore, the first message to the
monitor must be an MAuth.  If not, we assume it's a stray message and
just drop it.

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
mon: Monitor: dissociate msg handling from session & connection logic

Makes for simpler logic for patches to come.

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
mon: Monitor: only handle paxos msgs from a mon && if session has exec caps

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
mon: Monitor: reuse 'src_is_mon' bool on dispatch

Instead of making the same comparison twice with no gain at all.

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