Standardize naming of PipelineResult objects

[kennknowles]

Today:

PipelineResult is an interface returned by running a pipeline.
DataflowPipelineJob is the Dataflow implementation of that interface
FlinkRunnerResult is the Flink implementation
EvaluationContext is the Spark implementation
DirectPipelineResult is the DirectRunner implementation

Ideally, all the names would indicate that they are a PipelineResult, like the DirectRunner does.

Imported from Jira BEAM-726. Original Jira may contain additional context.
Reported by: bchambers.

[Subramanya-Veeregowda]

Hello, I'd like to work on this issue. I am assigning this issue myself.