mod_proxy_http2: start of some sort of flow control, mod_http2: keeping spare...

mod_proxy_http2: start of some sort of flow control, mod_http2: keeping spare allocator_t for slave connections around

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1732477 13f79535-47bb-0310-9956-ffa450edef68