mod_proxy: follow up to r1822849.

Get the help(er) of mod_http2 to determine how much connections should
be handled in the reslist by default (i.e. max_threads).



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1822878 13f79535-47bb-0310-9956-ffa450edef68