*) 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>.