Issues in event subscription server timer (thanks Mark Michelson for the report)

[nanang]

Reported issues:

• The timer TIMER_TYPE_UAS_TIMEOUT is not scheduled after receiving initial SUBSCRIBE, the timer will only be scheduled after receiving refresh SUBSCRIBE.

• Callback on_server_timeout() should not be invoked when subscription has already been terminated, sample scenario:
  1. receive SUBSCRIBE with an Expires header of 0
  2. the subscription state shifts to terminated, but the subscription destroy is delayed due to pending subscription transaction
  3. meanwhile, timer callback is called for TIMER_TYPE_UAS_TIMEOUT and on_server_timeout() callback is invoked.

[nanang]

In 4718:

Fix #1727:

• Start timer TIMER_TYPE_UAS_TIMEOUT when incoming subscription request is accepted (via pjsip_evsub_accept()).
• Kill any timer when subscription state is shifting into TERMINATED.