Data races in async-coap

An issue was discovered in the async-coap crate through 2020-12-08 for Rust. Affected versions of this crate implement Send/Sync for ArcGuard<RC, T> with no trait bounds on RC. This allows users to send RC: !Send to other threads and also allows users to concurrently access Rc: !Sync from multiple threads.

This can result in memory corruption from data race or other undefined behavior caused by sending T: !Send to other threads (e.g. dropping MutexGuard<T> in another thread that didn't lock its mutex).