Apply basic fusion to Java DirectRunner to avoid keeping all intermittent results in memory

Current java direct runner doesn't fuse transforms into steps. Instead, it almost executes each transform one by one. It results in memory pressure when any transform is high-fanout.

We already have a simple fusion logic in Java SDK(https://github.com/apache/beam/blob/master/runners/core-construction-java/src/main/java/org/apache/beam/runners/core/construction/graph/GreedyPipelineFuser.java). Work remaining here might be:

• Apply such fusion into DirectRunner
• Change the DirectRunner to be able run the fused steps.

I understand that DirectRunner doesn't expect processing large volume data and changing DirectRunner execution might be a fair amount of work.