Merge /httpd/httpd/trunk:r1900852,1900887

Merge  /httpd/httpd/trunk:r1900852,1900887

  *) mod_md: the `MDCertificateAuthority` directive can take more than one URL/name of
     an ACME CA. This gives a failover for renewals when several consecutive attempts
     to get a certificate failed.
     A new directive was added: `MDRetryDelay` sets the delay of retries.
     A new directive was added: `MDRetryFailover` sets the number of errored
     attempts before an alternate CA is selected for certificate renewals.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1900950 13f79535-47bb-0310-9956-ffa450edef68