res_pjsip: Add support for partial transport reload.

Some configuration items for a transport do not result in
the underlying transport changing, but instead are just
state we keep ourselves and use. It is perfectly reasonable
to change these items.

These include local_net and external_* information.

ASTERISK-29354

Change-Id: I027857ccfe4419f460243e562b5f098434b3d43a