[FLINK-1189] [streaming] Added cross operator

Conflicts:
	flink-addons/flink-streaming/flink-streaming-core/src/main/java/org/apache/flink/streaming/api/datastream/ConnectedDataStream.java
[FLINK-1189] [streaming] Temporal cross refactor

Conflicts:
	flink-addons/flink-streaming/flink-streaming-core/src/main/java/org/apache/flink/streaming/api/datastream/ConnectedDataStream.java
[FLINK-1175] [streaming] GroupBy updated to allow multiple field and keyselector groupings
[FLINK-1188] [streaming] Updated aggregations to work also on arrays by default
[streaming] Removed obsolete state objects from streaming-core
[FLINK-1185] [streaming] OperatorState and StateCheckpoint classes and implementations added for state checkpointing and rebalancing
[FLINK-1187] [streaming] StreamingRuntimeContext added with API support for registering fault tolerant state objects for the operators