Remove WebSockets Hijacker TODO. Websockets over HTTP/2 is a spec WIP.
Move enforcement of SETTINGS_MAX_CONCURRENT_STREAMS later.

Before it was enforced when processing the FRAME header, but as pointed
out on http2/http2-spec#649 , that means we were dropping any HPACK decoding
context in subsequent CONTINUATION frames.

Instead, move the check later to processHeaderBlockFragment (which
runs at END_HEADERS, whether in HEADERS or CONTINUATION).

And add a test using a CONTINUATION with decoder state to preserve.

This change also then is able to revert resetStream to its earlier
more paranoid behavior.