Merge r1822849, r1822858, r1822878, r1822879, r1822883, r1828485, r1828493 from trunk:

* When mod_http2 is loaded more then ThreadsPerChild backend connections can
  be useful as mod_http2 has an additional thread pool on top of
  ThreadsPerChild.
  But leave the default with ThreadsPerChild.


* Add some some comment why we do not limit hmax any longer


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).


mod_proxy: follow up to r1822849 and r1822878.

Does r1822878's "static" APR_RETRIEVE_OPTIONAL_FN work if, say, mod_proxy is
builtin but mod_http2 isn't?

Not worth taking the risk here since it's not a fast path...

Note: if this is an issue, I'm afraid it applies elsewhere too.


mod_proxy: follow up to r1822849 and r1822879.

Fix my maths, thanks Stefan and Rüdiger!


needs mod_http2.h


* Add missing CHANGES entry for revisions 1822849,1822858,1822878,1822879,1822883,1828485


Submitted ...