Another round of link serializer fixes

* Try to keep the separation between getLinkRoundTripData and the actual link
  handler

* Move pipe trick application to Util and use it from WTS to determine if the
  pipe trick is (still) applicable

Overall one more wt2wt and one more selser test are green. TODO: Add more
tests in this area!

Change-Id: Ie8bec17c75e9f074e458e508b5aa27c04065f3b8