pjsip: Ignore state changes from old transactions.

When we fail over to a new target we create a new transaction
and it becomes the current INVITE transaction. This does not
prevent the previous transaction from raising state changes
and causing the session to be prematurely disconnected if a
transport error occurs immediately.

This change backports a fix from PJSIP that eliminates the
incorrect state change and reduces when they would be raised
in the first place.

ASTERISK-27408

Change-Id: Id22d087591782eee31311753d11e7eca4b95ef34