rename tiddlers_etag to this_tiddlers_etag

Not much better, but at least somewhat better.

This is a common problem throughout the code, the word
tiddler is _everywhere_. We, however, do not want to
get all hungarian on the code.
Take all valid headers as keyword inputs to 304

This is just an experiment to get some feedback on
how it feels. This version remains backwards compatible and
specifically constrains itself to what the httpbis spec says
is valid.