Async TTM transformTokens: Refactored to use an accumChain obj.

* transformTokens effectively operates on an accumulator chain
  creating new nodes in the chain whenever a transformer generates
  an async chunk.  This chain code was distributed around in the
  function.  This patch consolidates all that code and state into
  the accumChain object.

* No change in parser tests results.

* Verified errorless and diffless RTing of en:Barack_Obama

Change-Id: I90e3dab23e1d71066bf53df609caa72db69da72f
Merge "Async TTM transformTokens: Refactored to use an accumChain obj."