Fix hash implementation to not use a global mutex

[pepone]

This PR fixes Reference and Endpoint classes hash implementation to use an instance mutex instead of a global mutex

[bernardnormier]

I don't think we should use any mutex in this code.

Either the hash is very cheap to compute and we compute it each time. Or it's somewhat expensive to compute (== bad hash) and we compute it in the constructor. As far as I can tell, this class is immutable.

[pepone]

I don't think we should use any mutex in this code.

I created a separate issue to fix this. I want to merge this as is as I'm getting rid of MutexPtrLock in my following PR.