DatastoreV1.write() Should Optionally Return PCollection<Void>

I'm trying to write to Datastore, wait for it to complete, then perform a task. In order to do this, the DatastoreV1.write() transform would need to return PCollection. Looking at the code, it returns PDone, and I can't find a workaround.

I propose adding an option to return PCollection. If I create a chained method like this then I think I can avoid breaking backwards compatibility:
```

DatastoreIO.v1().write().withReturn()

```

Alternatively, an entirely new write() method:
```

DatastoreIO.v1().writeWithReturn()
```

To illustrate the use case:
```

PCollection collection = someMethodToReturnPCollection();

PCollection datastoreWrites
= collection
.apply(
DatastoreIO.v1().write().withProjectId(projectId)
);

PCollection
otherCollection = ... // get the collection
otherCollection
.apply(
Wait.on(datastoreWrites)

)
.apply(
SomeOtherTransform()
);
```

I'll be creating a pull request soon to implement this.

Imported from Jira [BEAM-9491](https://issues.apache.org/jira/browse/BEAM-9491). Original Jira may contain additional context.
Reported by: alec.ferguson.