HTTP/2: always use temporary pool for processing headers.

HTTP/2: always use temporary pool for processing headers.

This is required for implementing per request timeouts.

Previously, the temporary pool was used only during skipping of
headers and the request pool was used otherwise.  That required
switching of pools if the request was closed while parsing.

It wasn't a problem since the request could be closed only after
the validation of the fully parsed header.  With the per request
timeouts, the request can be closed at any moment, and switching
of pools in the middle of parsing header name or value becomes a
problem.

To overcome this, the temporary pool is now always created and
used.  Special checks are added to keep it when either the stream
is being processed or until header block is fully parsed.