`Nullable<ClientType>` currently allows specifying either a concrete client
type or all client types, but does not support more flexible scoping options.
`ClientStorageScope` is designed to address that limitation, with future
support planned for special purpose scopes, such as locking the metadata file
shared by client directories.
This patch updates existing code, particularly around directory locks, to use
`ClientStorageScope` in place of `Nullable<ClientType>`.
Support for additional scope types will be added in a follow-up patch. This
patch focuses on transitioning to the new abstraction.
Differential Revision: https://phabricator.services.mozilla.com/D243630
74c99915a3