Async TTM: Reset activeAccum only after chunk is fully processed.

* transformTokens was updating activeAccum while processing the
  last token of the chunk rather than after it was fully
  processed.  Fixed now.

* It was also creating a new token accum when it was not needed
  (in non-async scenario).

* No change in parser test results.  This fixes parser test
  failures in chunky tokenizer mode which exercises async mode
  much more.

* Verified that en:Barack Obama RTs without errors.

Change-Id: I37c4a7d70b71325001704216038b9ea88c6b8b63