Use identifier where client library creates weak ID

[nerg4l]

Some client libraries only use the file size and the filename to create an ID. I consider this a weak ID as it is more likely that collisions can occur between sessions. SessionIdentifier could be used to generate stronger IDs by using a weak ID with the session identifier of the client. This way we can avoid collisions between sessions.

[nerg4l]

Driver	ID
Flow.js	size_filename
simple-uploader.js	size_filename
resumable-js	size_filename

It should be enough to only modify ResumableJsUploadDriver.