Add support for lockCheck

[rustyconover]

Check to see if a lock is held, but don't acquire it. Retries until the lock is no longer held.

[danielstjules]

Thanks for the PR! Taking a look now

[danielstjules]

I'm wondering if a slightly different interface would be more flexible:

• lock.isHeld(key, [fn]) returns a promise that resolves to a boolean indicating whether or not a lock is held given a particular key
• Have lock inherit from EventEmitter and add two events,lock.on('release'), and lock.on('acquire')

[danielstjules]

Nevermind, a release event wouldn't cover expiration in redis, and trying to resolve that with the current lock design would probably require key-space notifications being enabled. There's also probably no need for these events at all.

[danielstjules]

I can see lockCheck being abused to check state and conditionally acquire locks, which I think is an antipattern since such operations wouldn't be atomic. Could you give an example of your use case?

[danielstjules]

Gonna close for now due to inactivity, but thanks! :)