Ensure single should timeout

[JeffreyDevloo]

The ensure single currently waits for a process to clean itself up.
This does not happen in all cases:

• a worker dies
• A shell running decorated code is killed

It will also prevent the shell from mingling with tasks as tasks executed directly place the lock still but a worker restart will clear its entry.

[JeffreyDevloo]

It's better to keep all data contained within the key.
If multiple keys need to be adjusted all at once, you might have to rely on assert_range which is implemented in the newest pyrakoon version.