ServiceRegistry now stores a spec->references dict
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

ahmadshahwan/ipopo,ahmadshahwan/ipopo,tcalmant/ipopo