Reset AsyncTTM accumulators on hitting EOF.

* Async Token Transform Manager accumulators were not being
  reset on hitting EOF.  This left the TTM accumulator pipeline
  in an unusable state when it was reused for parsing new
  documents (ex: running parser tests).  This effect is not
  visible in the current master branch since the async abilities
  are not really being exercised.

  In the chunky-tokenizer branch where the tokenizer yields
  after each top-level block, tokens are processed a lot more
  asynchronously where this bug manifests in inter-pipeline
  events being lost leading to abrupt termination of the parser
  pipeline (and hence the parser test run).

* No change in parser test results.

Change-Id: I380f5124962686993a50806ea4b29349edf7e577