Add onAck delegation for Session

[slavikbialik]

Added the ability to make an onAck session delegation, so we'll be able to catch when we are getting a SIP ACK from PBX side.
This helps to reduce race conditions, because currently the only way to detect when a call is REALLY being answered is to listen on the session state when it becomes SessionState.ESTABLISHED, usually it becomes ESTABLISHED before the ACK arrives. But waiting for ACK and acting accordingly is more reliable way.

Note: this is my first PR here, I hope it is done according to your guidelines (which I couldn't find).

[john-e-riordan]

• A request to end a {@link Session} (incoming ACK).

should be

• A request to confirm a {@link Session} (incoming ACK).

Otherwise looks good to me. Thanks!

[slavikbialik]

• A request to end a {@link Session} (incoming ACK).

should be

• A request to confirm a {@link Session} (incoming ACK).

Otherwise looks good to me. Thanks!

Fixed!
This is what happens when you copy-paste 😅

[slavikbialik]

@john-e-riordan for some reason the CI failed as a result of a single test failure. Is there a way to make it re-run the tests without making any additional commit? Because the failed test should not fail because I just changed a comment 😓

[james-criscuolo]

Thanks again! Not sure what's happening with Travis, we saw it elsewhere too, will have to investigate.