h2: allow 100 streams by default

instead of 13, before the server has told how many streams it
accepts. The server can always reject new streams anyway if we go above
what it accepts.

Ref: #6826
Closes #6852

lib/http2.h

@@ -29,7 +29,7 @@
 
 /* value for MAX_CONCURRENT_STREAMS we use until we get an updated setting
    from the peer */
-#define DEFAULT_MAX_CONCURRENT_STREAMS 13
+#define DEFAULT_MAX_CONCURRENT_STREAMS 100
 
 /*
  * Store nghttp2 version info in this buffer, Prefix with a space.  Return