Merge r1872732, r1872777, r1874128, r1874190, r1874256, r1874542, r1874716, r1874720...

Merge r1872732, r1872777, r1874128, r1874190, r1874256, r1874542, r1874716, r1874720, r1874775, r1875465 from trunk:

Non-x86 testing seems reasonably reliable; note some known failures
currently.

Add Travis job with --enable-mods-static=reallyall.

Back off a little when svn export is timing out.

Add Travis job which runs under UBSan ("Undefined Behaviour Sanitizer").
mod_http2 disabled for now until https://github.com/icing/mod_h2/pull/194
is merged.

Add gcc 9 job, enable enhanced malloc debugging for pool-debug job.

Add an i386 build using a multilib packages and "gcc -m32".
(All other Travis builds are 64-bit arches currently)

Switch to using httpd-tests git repo.

Fix travis config validation warnings.

Update docs. The expr_string.t failure has not been seen since
the workaround was added AFAICT.  The async filter bug
is still breaking the tests regularly. [skip ci].

Add simple test for apxs to catch regression in r1872042.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1875466 13f79535-47bb-0310-9956-ffa450edef68