Change implementer_only to implementer.

Created by: tomprince

This removes the need to call registerAdapter on ZKAPAuthorizerStorageServer. It looks like that was needed because the interfaces for it were defined by implementer_only which explicitly ignored interfaces defined by parent classes. Looking at the history, I assume the use of implementer_only was to avoid implementing RIStorageServer via proxyForInterface. Given that the later is no longer used, implementer_only is no longer necessary.

This is extracted from #243.

Fixes #9 (closed)