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.