Allow user to set user data on Scan, include in log messages on server side

Create an api method, like ScannerOptions.setUserData(String), that will allow the user to set a unique id or some other data on the scan that will allow them to correlate scan messages across servers and clients. The user data should be included in all scan msgs and exceptions.

Start by locating ScannerOptions and tracing how scan messages and exceptions move between clients and servers. Define completion as a user-data API being available and the supplied data appearing in all scan messages and exceptions, with coverage for the affected paths.

Written by the indexing model from the issue text.