Ispn 4024/x site st alpha

ISPN-4024 Cross-Site State transfer: alpha version

https://issues.jboss.org/browse/ISPN-4024

Alpha version to review!!

This PR depends of:
* https://github.com/infinispan/infinispan/pull/2391
* https://github.com/infinispan/infinispan/pull/2328

Since it add new configuration parameters it would also depend of (*note* I didn't create a 7.0 parser but instead I changed the 6.0 parser because the configuration will be changed anyway):
* https://github.com/infinispan/infinispan/pull/2390

The documentation update is included in this PR.