doh2: ensure memory from unsent streams is freed

doh2: ensure memory from unsent streams is freed

The nghttp2 on_stream_close callback is only called for streams that are
properly closed. If we need to tear down the HTTP connection due to any
reason (e.g. IO error in underlying layer), some streams may not be
propely closed.

Due to HTTP/2 flow control, we may also wait indefinitely for the data
to be written. This can also cause the stream to never be properly
closed.

To handle these cases, a reference of allocated data is kept and we
ensure everything is freed once we're closing the http session.