http2: flow control and buffer improvements

- use bufq for send/receive of network data
- usd bufq for send/receive of stream data
- use HTTP/2 flow control with no-auto updates to control the
  amount of data we are buffering for a stream
  HTTP/2 stream window set to 128K after local tests, defined
  code constant for now
- elminiating PAUSEing nghttp2 processing when receiving data
  since a stream can now take in all DATA nghttp2 forwards

Improved scorecard and adjuste http2 stream window sizes
- scorecard improved output formatting and options default
- scorecard now also benchmarks small requests / second

Closes #10771

lib/bufq.h

@@ -201,10 +201,10 @@ ssize_t Curl_bufq_read(struct bufq *q, unsigned char *buf, size_t len,
  * Repeated calls return the same information until the buffer queue
  * is modified, see `Curl_bufq_skip()``
  */
-bool Curl_bufq_peek(const struct bufq *q,
+bool Curl_bufq_peek(struct bufq *q,
                     const unsigned char **pbuf, size_t *plen);
 
-bool Curl_bufq_peek_at(const struct bufq *q, size_t offset,
+bool Curl_bufq_peek_at(struct bufq *q, size_t offset,
                        const unsigned char **pbuf, size_t *plen);
 
 /**