Global lock fails when work takes longer than TTL

[vmachacek]

Once the lock is expired and work is still not finished another worker threads are free to start which is not ideal. My expectation is once I'm doing work no other worker is permitted to start.

[markjbrown]

thanks for reporting. I'm going through these and will take a look.

[markjbrown]

This will get fixed when #66 gets implemented.