Without this validation, test spuriously succeeds, but in fact it
doesn't.  This might occur if stream is closed with RST_STREAM or
GOAWAY.

We only change req_.upgrade_request once, so just using else is
suffice.

Currently, this does not do anything useful.

Use FieldStore::append_last_{header,trailer}_value instead.

Header field related functions are now gathered into FieldStore class.
This commit only handles request.  Subsequent commit will do the same
thing for response.

This fixes a crash in nghttpx when running on IPv6 only.

Disallow request from server, and response from client respectively.
When the violation is detected, return NGHTTP2_ERR_PROTO from
nghttp2_submit_request, nghttp2_submit_response,
nghttp2_submit_headers.

We also did some refactoring, and now self-dependency detection is
placed where it is only required.

Previously, we use session->next_stream_id to detect that given stream
ID was idle or not.  But this was suboptimal, since it was updated
when stream ID was assigned, and it did not necessarily mean that it
actually has been sent to the peer.  Now we introduced
session->sent_stream_id, which only updated when HEADERS/PUSH_PROMISE
has sent.  Using sent_stream_id instead of next_stream_id tightens
idle stream detection, and misbehaved peer which sends frame with
stream ID that has not been generated.

This commit also overhauls test code which involves opening streams.
Now we have some wrapper functions for nghttp2_session_open_stream()
which also take care of updating next_stream_id and
last_recv_stream_id.  They are crucial for some tests.