BigtableSession is never closed by Reader causing "ManagedChannel allocation site" exceptions

This isn't a major issue, but it is causing exceptions to be logged that may confuse someone who isn't familiar with grpc-java/netty.

#### Code example

The [BigtableSession created here](https://github.com/googleapis/java-bigtable-hbase/blob/master/bigtable-dataflow-parent/bigtable-hbase-beam/src/main/java/com/google/cloud/bigtable/beam/CloudBigtableIO.java#L583) is never closed.

#### Stack trace
```
java.lang.RuntimeException: ManagedChannel allocation site
at com.google.bigtable.repackaged.io.grpc.internal.ManagedChannelOrphanWrapper$ManagedChannelReference.(ManagedChannelOrphanWrapper.java:93)
at com.google.bigtable.repackaged.io.grpc.internal.ManagedChannelOrphanWrapper.(ManagedChannelOrphanWrapper.java:53)
at com.google.bigtable.repackaged.io.grpc.internal.ManagedChannelOrphanWrapper.(ManagedChannelOrphanWrapper.java:44)
at com.google.bigtable.repackaged.io.grpc.internal.AbstractManagedChannelImplBuilder.build(AbstractManagedChannelImplBuilder.java:517)
at com.google.bigtable.repackaged.com.google.cloud.bigtable.grpc.BigtableSession.createNettyChannel(BigtableSession.java:630)
at com.google.bigtable.repackaged.com.google.cloud.bigtable.grpc.BigtableSession.(BigtableSession.java:367)
at com.google.cloud.bigtable.beam.CloudBigtableIO$Reader.initializeScanner(CloudBigtableIO.java:583)
at com.google.cloud.bigtable.beam.CloudBigtableIO$Reader.start(CloudBigtableIO.java:573)
at org.apache.beam.runners.dataflow.worker.WorkerCustomSources$BoundedReaderIterator.start(WorkerCustomSources.java:607)
at org.apache.beam.runners.dataflow.worker.util.common.worker.ReadOperation$SynchronizedReaderIterator.start(ReadOperation.java:361)
at org.apache.beam.runners.dataflow.worker.util.common.worker.ReadOperation.runReadLoop(ReadOperation.java:194)
at org.apache.beam.runners.dataflow.worker.util.common.worker.ReadOperation.start(ReadOperation.java:159)
at org.apache.beam.runners.dataflow.worker.util.common.worker.MapTaskExecutor.execute(MapTaskExecutor.java:77)
at org.apache.beam.runners.dataflow.worker.BatchDataflowWorker.executeWork(BatchDataflowWorker.java:417)
at org.apache.beam.runners.dataflow.worker.BatchDataflowWorker.doWork(BatchDataflowWorker.java:386)
at org.apache.beam.runners.dataflow.worker.BatchDataflowWorker.getAndPerformWork(BatchDataflowWorker.java:311)
at org.apache.beam.runners.dataflow.worker.DataflowBatchWorkerHarness$WorkerThread.doWork(DataflowBatchWorkerHarness.java:140)
at org.apache.beam.runners.dataflow.worker.DataflowBatchWorkerHarness$WorkerThread.call(DataflowBatchWorkerHarness.java:120)
at org.apache.beam.runners.dataflow.worker.DataflowBatchWorkerHarness$WorkerThread.call(DataflowBatchWorkerHarness.java:107)
at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
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:834)
```