Java API V2 Session Upload

This API is kind of a mess right now.  We need to sort some things out to make it clearer, but for now, this might work:

```

DbxClientV2 client = ...

uploader = client.files.uploadSessionStart();

String sessionId;

try {

    uploader.getBody().write(first_chunk_of_upload)

    sessionId = uploader.finish()

} finally {

    uploader.close()

}

uploader = client.files.uploadSessionAppend(sessionId, offset)

try {

    uploader.getBody().write(second_chunk_of_upload)

    uploader.finish()

} finally {

    uploader.close()

}

cursor = UploadSessionCursor(sessionId, offset);

FileMetadata md = client.files.uploadSessionFinishBuilder(cursor, ...).run(third_chunk_of_upload);

```

I don't have time to test the code right now.  Lemme know if you run into any issues and I'll take a closer look.