Add the ability to update external sources for a running repository without restarting the entire repository

When configured with external sources, the ModeShape repository stores very little internal state and mostly relies on each of the configured sources to provide it with data and information.

As such, when a new external source is added or the attributes of an existing one are modified, it should be possible to reflect these changes via an update operation for a running repository without one having to restart the entire repository.

This is feature is probably most likely to be used via the Wildfly admin console, where one could easily make the above changes.