Premature termination of REFER (call transfer) subscription

[nanang]

Scenario:

1. A calls B.
2. B transfer to C.
3. C reject the incoming call.
4. immediately, B transfer to C again.
5. B will see log line that REFER subscription is terminated, and any NOTIFY messages from A will be responded with 481 (Subscription already terminated). Note: A will send BYE to B after receiving 481/NOTIFY response from B, so if C rejects the transfer again, the original call A-B will be gone, but if C accepts the transfer, call A-C will be established (call transfer succeeds).

After investigation, the new subscription for the second REFER is actually created, while the old one has not been destroyed, but the Event Subscription module somehow sees the old and terminated subscription (instead of the new one) for the second REFER.

[nanang]

In 4447:

Close #1647: updated subscription lookup in on_new_transaction() to skip subscription with NO_EVENT_ID and already terminated.