Delayed sending of ACK request (using on_send_ack() ) may prematurely send the ACK

[ming]

Scenario:

1. send INVITE
2. recv 200/OK
3. app implements on_send_ack() and calls pjsip_inv_create_ack(), but have not sent it.
4. recv retransmission of 200/OK
5. PJSIP auto answers the ACK

The doc of on_send_ack() says:

     * Application creates the ACK request
     *
     * Once it has sent the ACK request, the framework will keep 
     * this ACK request in the cache. Subsequent receipt of 2xx response
     * will not cause this callback to be called, and instead automatic
     * retransmission of this ACK request from the cache will be done
     * by the framework.

But actually pjsip_inv_create_ack() is the one that saves inv->last_ack. So the auto transmission already happens in inv_send_ack() of sip_inv.c, even though app hasn't sent the ACK.

[ming]

In 5931:

Fixed #2175: Make sure that last ACK has been sent before retransmitting it.

Also update the doc that the callback can now be called more than once.