WTS: Code cleanup + minor simplification.

* Get rid of chunkCB in serializer state and pass in a cb into all
  serializeNode calls. This keeps code readable and eliminates the
  extra piece of state in 'SerializerState'.

* This also lets us clean up WTS kickoff code.

* Added state.onSOL output to tracing output for every node.

Change-Id: I704221b2e349115c500c231c67f1b276ae47523a
Merge "WTS: Code cleanup + minor simplification."