Trace categories settings and configuration

[tusmester]

Currently if the developer provides a list of trace categories (System, Index, Repository, etc.) to switch ON, the system does not take the settings stored in the repository into account.

TODO

• [ ] merge trace settings sources: config + settings (with OR operation)
• [ ] refactor the API to be more straightforward: how to define categories in Startup?
• [ ] documentation

Be aware that the repository is not always started when this API is called, need to be careful about accessing the settings content.

SnTraceConfigurator.UpdateCategories(string[] categoryNames)