Follow-up to https://github.com/scylladb/scylla/pull/6916.
- Fixes wrong usage of `resource_manager::prepare_per_device_limits`,
- Improves locking in `resource_manager` so that it is more safe to call its methods concurrently,
- Adds comments around `resource_manager::register_manager` so that it's more clear what this method does and why.
Closes#7660
* github.com:scylladb/scylla:
hints/resource_manager: add comments to register_manager
hints/resource_manager: fix indentation
hints/resource_manager: improve mutual exclusion
hints/resource_manager: correct prepare_per_device_limits usage