Fix crash when buffer was reallocated after read_length_callback

Added test for this crash.
Check buffer capacity explicitly
Add test to submit DATA frame twice

This commit adds test to submit DATA frame twice and fixes the bug
that 2nd DATA is not sent.
python: Fix build error
nghttpx: Don't consume response data in downstream on_stream_close_callback

Even after on_stream_close_callback, Http2DownstreamConnection is
still alive and upstream keeps sending response to the client.  The
consumed bytes are processed normally (data_source_read_callback) and
also we have a code to consume all allocated bytes for
Http2DownstreamConnection object when it is deleted.  This means that
we don't need to and should not consume response data in downstream
on_stream_close_callback.  If we do, we may get assertion error in
Http2DownstreamConnection::resume_read().
python: Add version