Reworking of Iteration Synchronization, Accumulators and Aggregators

This pullrequest includes:
Iteration superstep synchronization through RPC and JobManager
Unification of Accumulators and Aggregators (removal of former Aggregators)
Adjusted Testcases accordingly

A detailed description can also be found on Jira: https://issues.apache.org/jira/browse/FLINK-951

Currently the testcase "KMeansIterativeNepheleITCase" sometimes fails. I think this is connected to this issue: https://issues.apache.org/jira/browse/FLINK-909 and needs to be resolved in a following pull request.