GH-427: Read initial ACK on channel open prior to direct stream upload & close streams prior to exit code handling

[TerraNibble]

Fixes GH-427.

• Ensures that the initial ACK sent upon creating the channel is processed prior to sending the payload stream.
• Channel IO streams are first closed to trigger EOF prior to handling the exit code.
• DefaultScpClient.upload(InputStream local, ...) no longer hits exit status timeout (5 seconds by default) on each invocation.
• This can be observed by running ScpTest.testStreamsUploadAndDownload.