CVE-2020-36460: Data races in model

Shared data structure in model crate implements Send and Sync traits regardless of the inner type. This allows safe Rust code to trigger a data race, which is undefined behavior in Rust.

Users are advised to treat Shared as an unsafe type. It should not be used outside of the testing context, and care must be taken so that the testing code does not have a data race besides a race condition that is expected to be caught by the test.