Cleanup bigtable open resources on teardown

[RustedBones]

We've noticed that a failure to process a bigquery Mutation (for instance due to a coder exception), results with the following error in the logs:

Batcher was not closed properly!!! Make sure to call close().
java.lang.RuntimeException: Batcher allocation site
	at com.google.api.gax.batching.BatcherImpl$BatcherReference.<init>(BatcherImpl.java:552)
	at com.google.api.gax.batching.BatcherImpl.<init>(BatcherImpl.java:210)
	at com.google.cloud.bigtable.data.v2.stub.EnhancedBigtableStub.newMutateRowsBatcher(EnhancedBigtableStub.java:690)
	at com.google.cloud.bigtable.data.v2.BigtableDataClient.newBulkMutationBatcher(BigtableDataClient.java:1113)
	at com.google.cloud.bigtable.data.v2.BigtableDataClient.newBulkMutationBatcher(BigtableDataClient.java:1080)
	at org.apache.beam.sdk.io.gcp.bigtable.BigtableServiceImpl$BigtableWriterImpl.<init>(BigtableServiceImpl.java:501)
	at org.apache.beam.sdk.io.gcp.bigtable.BigtableServiceImpl.openForWriting(BigtableServiceImpl.java:131)
	at org.apache.beam.sdk.io.gcp.bigtable.BigtableServiceImpl.openForWriting(BigtableServiceImpl.java:93)
	at org.apache.beam.sdk.io.gcp.bigtable.BigtableIO$BigtableWriterFn.startBundle(BigtableIO.java:1248)
	at org.apache.beam.sdk.io.gcp.bigtable.BigtableIO$BigtableWriterFn$DoFnInvoker.invokeStartBundle(Unknown Source)
	at org.apache.beam.fn.harness.FnApiDoFnRunner.startBundle(FnApiDoFnRunner.java:797)
	at org.apache.beam.fn.harness.data.PTransformFunctionRegistry.lambda$register$0(PTransformFunctionRegistry.java:116)
	at org.apache.beam.fn.harness.control.ProcessBundleHandler.processBundle(ProcessBundleHandler.java:530)
	at org.apache.beam.fn.harness.control.BeamFnControlClient.delegateOnInstructionRequestType(BeamFnControlClient.java:150)
	at org.apache.beam.fn.harness.control.BeamFnControlClient$InboundObserver.lambda$onNext$0(BeamFnControlClient.java:115)
	at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:515)
	at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
	at org.apache.beam.sdk.util.UnboundedScheduledExecutorService$ScheduledFutureTask.run(UnboundedScheduledExecutorService.java:163)
	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
	at java.base/java.lang.Thread.run(Thread.java:829)

The bigtableWriter and serviceEntry resources are allocated in the StartBundle and only closed in the FinishBundle. If an exception is raised during the ProcessElement, FinishBunlde is not called, resulting in a memory leak when retrying the step.

This change makes sure to close those resources during the TearDown phase

[github-actions]

Assigning reviewers. If you would like to opt out of this review, comment assign to next reviewer:

R: @bvolpato for label java.
R: @ahmedabu98 for label io.
R: @mutianf for label bigtable.

Available commands:

• stop reviewer notifications - opt out of the automated review tooling
• remind me after tests pass - tag the comment author after tests pass
• waiting on author - shift the attention set back to the author (any comment or push by the author will return the attention set to the reviewers)

The PR bot will only process comments in the main thread (not review comments).

[Abacn]

lgtm, thank you!