add DistributedGroupByConsumer
adapt TransportExecutor to be able to create and run tasks dynamically

and streamlined the TAsk interface and usage
Merge pull request #1474 from crate/m/task_executor

adapt TransportExecutor to be able to create and run tasks dynamically
throw meaningful errors if expression is unsupported
Merge remote-tracking branch 'origin/relations' into b/merge_master_relations

Conflicts:
	sql/src/main/java/io/crate/planner/Planner.java