With #6779 we removed unnecessary allocations from the timerwheel by
wrapping it in an `std::sync::RwLock`. Since the `Mutex` used in this
part of the project uses an abstraction in `loom::sync::Mutex` to get
rid of the poisoning aspects of `std::sync::Mutex` the same should
probably be done for the used read-write lock struct.
This commit introduces an abstraction to get rid of the poisoning
aspects of `std::sync::RwLock` by introducing a wrapper to the
`loom::sync` module similar to `loom::sync::Mutex`.
Refs: #6779
Timo