Mutex unlocks should only be allowed to be called once

[karimsa]

Description

Currently, Mutexes, Semaphores, & RWMutexes will return an unlock callback which

Steps to Reproduce (for bugs)

1. Lock a semaphoer & store its callback function.
2. Lock a semaphore in a separate process & store its callback function.
3. In process P1, call the unlock function twice.

Expected: The second call should either fail or do nothing.
Current: The lock will be unlocked for the second process.

Your Environment (for bugs)

• Package version (exact): 0.5.2
• Operating system: any
• Node.js version: 6.x.x
• npm version: 6.x.x