Add config and init logic for updating online GC review delay

Related to #299 (closed). This is the last MR needed to resolve this issue. It adds the user facing configuration and the update logic.

Additionally, for reusability, this MR creates a new registry/internal and registry/internal/testutil packages were common code (Clock interface) and test util methods are now located (moved from registry/gc). As a consequence, registry/gc has been refactored to reuse these shared types, functions and mocks.