nghttpx: Call upstream resume_read after sending pending request to backend

With the combination of HTTP/1 upstream and HTTP/2 downstream,
downstream tells SHRPX_NO_BUFFER while connecting to the backend
server.  Previously, we did not call upstream resume_read and upload
was blocked.  This commit now calls upstream resume_read to unblock.
This commit also remove pending output buffer size of Http2Session
when calculating downstream connection's buffer is full.  This is
desirable since we only operate resume_read by stream basis.
nghttpx: Add more handling situation where response ends before request
Fix memory leak around stream->data_item

Previously we missed the case where stream->data_item is not deleted
and it caused leak.  Now stream->data_item is properly deleted when
session is deleted.  We decided not to delete data_item in
nghttp2_stream_free() since we need nghttp2_session to decide whether
data_item should be deleted or not there.
Add missing NGHTTP2_ERR_BAD_PREFACE to nghttp2_strerror