Find the alive incoming entry on disconnect. (#6320)

When a peer in `Incoming` state disconnects, the "alive" entry
in the `incoming` list for that peer must be updated (set to `false`).
Currently the entry that is updated may be an earlier entry for the
same peer that is already no longer alive. This can happen if a
peer repeatedly connects (incoming) and disconnects between invocations to
`poll()` of the behaviour.