Fix Rust build

[oowekyala]

Fixes the build issues found after merging #488 into master.

[oowekyala]

Everything looks green. For reference, the bug was caused by two threads trying to access the same physical action concurrently. This is a data race, but the smart pointer we use detected that and panicked. I replaced it with a mutex. I'm not sure how to reproduce this in a test case, as the faulty behavior is only observable in a certain interleaving of threads. That's also why the test failed only unpredictably.

[Soroosh129]

Everything looks green. For reference, the bug was caused by two threads trying to access the same physical action concurrently. This is a data race, but the smart pointer we use detected that and panicked. I replaced it with a mutex. I'm not sure how to reproduce this in a test case, as the faulty behavior is only observable in a certain interleaving of threads. That's also why the test failed only unpredictably.

Would you please create an issue for this?

[lhstrh]

I think this PR already fixes the issue @oowekyala describes here.