pjsip_transport_events: handle multiple addresses for a domain

pjsip_transport_events: handle multiple addresses for a domain

The key used for transport monitors was the remote host name for the
transport and not the remote address resolved for this domain.

This was problematic for domains returning multiple addresses as several
transport monitors were created with the same key.

Whenever a subsystem wanted to register a callback it would always end
up attached to the first transport monitor with a matching key.

The key used for transport monitors is now the remote address and port
the transport actually connected to.

Fixes: #932
(cherry picked from commit d1ed1018c426726976ff8a7132072f1f5ae161df)