FileChannel instance should be closed in ArtifactServiceStager#StagingCallable#get

```

FileChannel channel = new FileInputStream(file).getChannel();
ByteBuffer readBuffer =
ByteBuffer.allocate(bufferSize);
while (!responseObserver.isTerminal() && channel.position()
< channel.size()) {
readBuffer.clear();
channel.read(readBuffer);

```

The channel should be closed before returning from get()

Imported from Jira [BEAM-3935](https://issues.apache.org/jira/browse/BEAM-3935). Original Jira may contain additional context.
Reported by: yuzhihong@gmail.com.