Split mod_cluster code base into container integration and native repositories

Time has shown the Java and C codebases need to be split. Among other reasons:

• commits almost never span across Java and C code
• the 2 parts do not live the same lifecycle yet share the release versions
• simplification of CI/testing
• donation of the split native part to apache usptream

The repository will be split into to repos:

• https://github.com/modcluster/mod_cluster – the original repository will hold all previous versions in entirety 1.0.x, 1.1.x, 1.2.x, 1.3.x and all releases + master (2.x) will continue to only containe Java container integration
• https://github.com/modcluster/mod_proxy_cluster – will contain all version 2.x development of the native parts