http2: Read data left in connection buffer after pause

Previously when we do pause because of out of buffer, we just throw
away unread data in connection buffer.  This just broke protocol
framing, and I saw occasional FRAME_SIZE_ERROR.  This commit fix this
issue by remembering how much data read, and in the next iteration, we
process remaining data.

lib/http.h

@@ -191,6 +191,8 @@ struct http_conn {
   sending send_underlying; /* underlying send Curl_send callback */
   recving recv_underlying; /* underlying recv Curl_recv callback */
   char *inbuf; /* buffer to receive data from underlying socket */
+  size_t inbuflen; /* number of bytes filled in inbuf */
+  size_t nread_inbuf; /* number of bytes read from in inbuf */
   /* We need separate buffer for transmission and reception because we
      may call nghttp2_session_send() after the
      nghttp2_session_mem_recv() but mem buffer is still not full. In