[repo] move repo.gc_stats to Service API (closes #2951068) We currently have a method on the repo that anyone can call. `call_service` gives a similar thing, modulo: * can be restricted access to manager only (thanks to selection on services), * less unrelated code on the repository class, * no need to fetch a reference to a repo object from the client side * (`req._cnx.repo` is not an API and config.repository() is on its way out). The old way to access this information (repo.gc_stats()) is deprecated.

option_group_changed('port', 'main', 'web')
option_group_changed('query-log-file', 'main', 'web')
option_group_changed('profile', 'main', 'web')
option_group_changed('max-post-length', 'main', 'web')