ValueState for Dataflow runner and MapState for others

[Dmitry Minaev]

Hi everyone,

Since Dataflow doesn't support MapState (
https://issues.apache.org/jira/browse/BEAM-1474) I'm thinking of using
ValueState with a Map<> inside it. Is it a good idea? Here is an example
code:
```
@StateId("myValueStore")
private final StateSpec<ValueState<Map<String, String>>> valueStoreSpec =
StateSpecs.value(MapCoder.of(StringUtf8Coder.of(), StringUtf8Coder.of()));

@ProcessElement
public void processElement( ProcessContext
context, @StateId("myValueStore") MapState<String, String> valueStore) {
    ...
}
```

I'd like to support other runners as well (e.g. FlinkRunner) and it seems
to be more efficient to use MapState in cases where I need to store a map
of values. So I'm thinking of the way to use MapState and ValueState for
different runners.

I understand how to get a runner in runtime via pipeline options but I'm
not sure how to approach defining (and using) different StateSpec for
different runners.

Here is a sample code for MapState:
```
@StateId("myMapStore")
private final StateSpec<MapState<String, String>> mapStoreSpec =
StateSpecs.map(StringUtf8Coder.of(), StringUtf8Coder.of());

@ProcessElement
public void processElement( ProcessContext context,
        @StateId("myMapStore") MapState<String, String> mapStore) {
    ...
}
```

Any ideas?

Thank you,
Dmitry