ServiceRegistry now stores a spec->references dict

This avoids to walk through the whole service registry when we can
pre-select a subset of references according to the requested
specification.
+ the search for a single reference now stops at the first match

=> This speeds up get_service_refence() to 5x
Small improvement of get_service()

- Replaced if .. in .. by a try-except KeyError (avoids a double lookup)
- Removed the temporary variable "imports"
Comment: find_svc_refs must not return None if no class nor filter given

It means that we require a copy of the whole registry