h2load: Effectively disable flow control by setting large window size

Previously h2load used default flow control window as described in
HTTP/2 and SPDY specification.  The window size is 64KiB, which is a
bit small, and cannot utilize full server performance when response
size is not too small.  Basically, we do this kind of benchmarking
test to measure server's throughput, and optimal performance.  Smaller
window certainly degrades performance even in local testing because
server is so fast that it has to wait for WINDOW_UPDATE from h2load.
To make default behaviour suitable for peak performance test, we
decided to disable flow control in h2load by setting large enough
window size.

Most users used h2load without -w or -W options, so they were
implicitly throttled by flow control and the result was affected by
that negatively.  Now flow control is disabled by default, the result
may improve depending on the implementations.

doc/h2load.h2r

@@ -49,6 +49,15 @@ time for request
     The fraction of the number of requests within standard deviation
     range (mean +/- sd) against total number of successful requests.
 
+FLOW CONTROL
+------------
+
+h2load sets large flow control window by default, and effectively
+disables flow control to avoid under utilization of server
+performance.  To set smaller flow control window, use :option:`-w` and
+:option:`-W` options.  For example, use ``-w16 -W16`` to set default
+window size described in HTTP/2 and SPDY protocol specification.
+
 SEE ALSO
 --------
 

src/h2load.cc

@@ -69,7 +69,7 @@ namespace h2load {
 
 Config::Config()
     : data_length(-1), addrs(nullptr), nreqs(1), nclients(1), nthreads(1),
-      max_concurrent_streams(-1), window_bits(16), connection_window_bits(16),
+      max_concurrent_streams(-1), window_bits(30), connection_window_bits(30),
       no_tls_proto(PROTO_HTTP2), data_fd(-1), port(0), default_port(0),
       verbose(false) {}
 
@@ -974,11 +974,13 @@ Options:
   -w, --window-bits=<N>
               Sets the stream level initial window size to (2**<N>)-1.
               For SPDY, 2**<N> is used instead.
+              Default: )" << config.window_bits << R"(
   -W, --connection-window-bits=<N>
               Sets  the  connection  level   initial  window  size  to
               (2**<N>)-1.  For SPDY, if <N>  is strictly less than 16,
               this option  is ignored.   Otherwise 2**<N> is  used for
               SPDY.
+              Default: )" << config.connection_window_bits << R"(
   -H, --header=<HEADER>
               Add/Override a header to the requests.
   -p, --no-tls-proto=<PROTOID>