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

tests/http/testenv/env.py

@@ -63,6 +63,8 @@ class EnvConfig:
         self.config = DEF_CONFIG
         # check cur and its features
         self.curl = CURL
+        if 'CURL' in os.environ:
+            self.curl = os.environ['CURL']
         self.curl_props = {
             'version': None,
             'os': None,