Add support for MapReduceTask combiners

Infinispan 5.2 added the ability to specify a Reducer that would be used
to reduce the values on the local node before the global reduce phase is
executed. This commit adds support for specifying this option in the
MapReduceStage parameters and implements support for them in the
MapReduceCapable interface and InfinispanMapReduce class.
Updated Infinispan version for JDG 6.2 plugin
Fix: in TX context the entries written in one transactions can be committed in different order
JGroups plugins for different versions. Removed configuration properties.
Support for batching (replaces transactions in the test)