Replace `RefCell` with `AtomicRefCell` for structs implementing Actor,
making them `Sync`.
Consolidate `register` and `register_later` into a single function,
removing the need to wait for a loop before accessing newly created
actors.
Now `ActorRegsitry` has improved locking. Instead of locking the entire
struct, each member can be locked separately. Additionally, since `find`
now returns `Arc`, we can `find` and `register` multiple actors
depending on each other, since the lock is only needed for the operation
and we can keep the reference after that.
Depends on: #41741, #41744
Testing: Manual testing
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Signed-off-by: eri <eri@igalia.com>
Co-authored-by: Martin Robinson <mrobinson@igalia.com>
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