Merge r1852038, r1852101 from trunk:

mod_http2: enable re-use of slave connections again.


mod_http2: fixed slave connection keepalives counter.

Submitted by: icing
Reviewed by: icing, ylavic, jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1855406 13f79535-47bb-0310-9956-ffa450edef68