Continuation of MODCLUSTER-494.

Time has shown that this was a good decision but we also need to move the remaining supported 1.3.x branch to the mod_proxy_cluster repository.

Moreover, the no-longer developed 1.3.x container-integration code (which has been superseded by 1.4.x and later branches) should be removed from this branch as it's confusing that new releses do not bring any changes to this code and don't fix any bugs.

No tags will be moved.