Support SetState in Dataflow runner

[kennknowles]

Imported from Jira BEAM-1479. Original Jira may contain additional context. Reported by: kenn.

[scwhittle]

We could do so by implementing on top of existing MultiMapState similiar to how it is done for the fnapi harness

https://github.com/apache/beam/blob/master/sdks/java/harness/src/main/java/org/apache/beam/fn/harness/state/FnApiStateAccessor.java#L370