Initial XML unification work

* Created a new parser for Infinispan 7.0
* Model the parser based on server configuration parsing
* Changed virtual nodes parsing for segments
* Order attributes and elements alphabetically
* Make ClusteringConfigurationBuilder.cacheMode() public in order to
be able to inspect what the cache mode is while parsing and be able
to configure async marshalling accordingly.
* Add unit test for testing parsing of unified XML, needs more asserts
* Separated JGroups stack handling to a separate element and add a new
stack-file element for stacks that are defined via an external file.
* Added file-store and commons cache store element/attribute support.
* Added support cross-site backup configuration.
Minor XML adjustment