Each thread has its own rados context. The thead pool, when used will allow better
parallelism of asynchronous requests.

This required some rework on the backend creations as the threads will need logging.

New URLs are rados://user@pool and file:///tmp/XXXXX. Otherwise, we fall back to interpreting
the URL as a bare path.

Implemented unit tests for storage classes and archive routes.

License comment is no longer filtered out by doxygen when generating web pages of the individual source files.