add shutdown and start mechanics to windmill streams

[m-trieu]

Start and closing Windmill streams are currently via halfClose() and on stream creation. Implementations were previously created and returned in a "started" state usually after the stream has already sent the initial headers to open the connection to the backend servers.

Starting in the current state prevents us from being able to start the stream "lazily". And closing allows other blocking stream operations to prevent streams from being able to be closed (stalling at times up to 10-20 minutes).

• Add start() flexibility to the WindmillStream API by allowing external callers to start the stream themselves.
• Add shutdown() capability to allow the stream to receive a shutdown signal, that is idempotent and does not block (or is blocked by) other blocking stream operations.

This is especially important in direct path mode where the user worker manages the fan out to the backend.

in terms of implementation, similar to WindmillStream.shutdown(), WindmillStream.start()'s behavior will only execute once during the lifetime of the WindmillStream object. Subsequent calls to start() and shutdown() will do nothing.

R: @arunpandianp @scwhittle

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[m-trieu]

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[m-trieu]

back to you @arunpandianp Thanks!

[m-trieu]

back to you @arunpandianp thanks!

[m-trieu]

back to you @arunpandianp @scwhittle thanks!

[m-trieu]

Run Java Precommit

[m-trieu]

failures seem to be unrelated

[m-trieu]

Back to you! Thanks

@arunpandianp @scwhittle

[arunpandianp]

Like how there was a race between populating pending maps and shutdown, I think there is a race between mutating/accessing batches and shutdown in GrpcGetDataStream. Other pieces look okay to me.

[m-trieu]

@arunpandianp @scwhittle back to you thanks!

[m-trieu]

Done back to you @arunpandianp Thanks!

[m-trieu]

Run Java Precommit

[m-trieu]

back to you @scwhittle @arunpandianp thanks!

[m-trieu]

back to you @arunpandianp @scwhittle thanks

[m-trieu]

back to you thanks @scwhittle

[m-trieu]

back to you @scwhittle thank you!

[m-trieu]

still need to add some more tests to DirectStreamObserverTest addressed the other comments

thanks! @scwhittle

[m-trieu]

back to you @scwhittle thanks!

[Abacn]

GrpcGetDataStreamTest.java fails due to a second test added to the test suite:

java.io.IOException: name already registered: Fake server for GrpcGetDataStreamTest
	at io.grpc.inprocess.InProcessServer.registerInstance(InProcessServer.java:89)
	at io.grpc.inprocess.InProcessServer.start(InProcessServer.java:80)
	at io.grpc.internal.ServerImpl.start(ServerImpl.java:185)
	at io.grpc.internal.ServerImpl.start(ServerImpl.java:94)
	at org.apache.beam.runners.dataflow.worker.windmill.client.grpc.GrpcGetDataStreamTest.setUp(GrpcGetDataStreamTest.java:80)

[Abacn]

even though the naming fix, it still failed in internal import