This commit makes db::hints::manager store service::storage_proxy
as a reference instead of a seastar::shared_ptr. The manager is
owned by storage proxy, so it only lives as long as storage proxy
does. Hence, it makes little sense to store the latter as a shared
pointer; in fact, it's very confusing and may be error-prone.
The field never changes, so it's safe to keep it as a reference
(especially because copy and move constructors of db::hints::manager
are both deleted). What's more, we ensure that the hint manager
has access to storage proxy as soon as it's created.
The same changes were applied to db::hints::resource_manager.
The rationale is the same.
18a2831186