Coalesce connections

For #490.  The changes here are fairly minimal.  Turns out that coalescing was already just a "MAY".

I'm still waiting on a resolution to #496 here for the "Not Authoritative" status code.

Another security consideration here is that origin servers can potentially probe the header tables of each other in this case.  I want to write some text on that separately, because the current text in HPACK is inadequate to really understand the scope of this issue.