Fixes https://github.com/pion/webrtc/issues/2089

When a retranmission from the remote side arrives after
the handshake is complete, the `finish` routine
puts it back into retransmit loop.

With Chrome, this fails after 15 seconds.
Firefox does not error out though.

Testing:
---------
- Tested with Firefox and Chrome with long delay
(500 ms up and down) in network link conditioner.
- Tested the above with no introduced delays too.
- Added test for slow server.