WTS: Don't buffer accumulated separator text across a DOM subtree.

* The serializer was buffering separator text from child-nodes
  in some cases, and emitting in other cases. This might have
  been related to how 'handleSeparatorText' dealt with single-child
  and multi-child nodes.

* Simplified code by having uniform text/comment handling independent
  of # of children + always emitting accumulated separator text
  rather than buffering it across the dom-subtree boundary.

* Fixes RTing of the following snippet (the comment was being lost)
-----------
{|
| <!--a-->
| b
|}
-----------

* Improvement in html2html output of failed tests + fewer DSR
  inconsistency warnings in html2html tests.

* Eliminates several semantic errors from:
  - en:List of airports in Alabama

Change-Id: Ica2d4142ec20821daf4af64f7e91b17c14699485