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)