netty4 only changes (w/o MLSS stuff)
minor refactoring in groupcomm operators
Merge branch 'master' into sergiym_MLSS_netty4

Conflicts:
	reef-io/src/main/java/com/microsoft/reef/io/data/loading/api/DataLoader.java
	reef-io/src/main/java/com/microsoft/reef/io/network/group/GroupCommMessage.java
	reef-io/src/main/java/com/microsoft/reef/io/network/group/impl/operators/basic/AllGatherOp.java
	reef-io/src/main/java/com/microsoft/reef/io/network/group/operators/AbstractGroupCommOperator.java
	reef-io/src/main/java/com/microsoft/reef/io/network/group/operators/AllGather.java
	reef-io/src/main/java/com/microsoft/reef/io/network/group/operators/Broadcast.java
	reef-io/src/main/java/com/microsoft/reef/io/network/group/operators/Gather.java
	reef-io/src/main/java/com/microsoft/reef/io/network/group/operators/GroupCommOperator.java
	reef-io/src/main/java/com/microsoft/reef/io/network/group/operators/Reduce.java
	reef-io/src/main/java/com/microsoft/reef/io/network/group/operators/ReduceScatter.java
	reef-io/src/main/java/com/microsoft/reef/io/network/group/operators/Scatter.java