Enforce Mutex.unlock contract

[qwwdfsad]

Background

Mutex API allows to pass an optional and arbitrary owner token to its methods, lock and unlock. This token can be used as a piece of additional debugging information to ensure Mutex ownership and catch programmatic errors early.
If a user tries to unlock a mutex with an owner different from the one it was locked with, the mutex will throw IllegalStateException.

Bug

The following call sequence is currently allowed by the implementation:

mutex.lock(someOwner)
mutex.unlock() // No owner at all

This behaviour becomes even trickier when the owner is operated as Any? in the user code, increasing the chances for null to sneak in.

Proposed change

Throw an IllegalStateException when no owner was supplied to unlock call for the mutex locked with the non-null owner.

[qwwdfsad]

We've discussed it and decided that the potential disturbance does not worth it