Merge of /httpd/httpd/trunk:r1903677

Merge of  /httpd/httpd/trunk:r1903677

  *) mod_md: a new directive `MDStoreLocks` can be used on cluster
     setups with a shared file system for `MDStoreDir` to order
     activation of renewed certificates when several cluster nodes are
     restarted at the same time. Store locks are not enabled by default.

     Restored curl_easy cleanup behaviour from v2.4.14 and refactored
     the use of curl_multi for OCSP requests to work with that.
     Fixes <https://github.com/icing/mod_md/issues/293>.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1903678 13f79535-47bb-0310-9956-ffa450edef68