Improve locks

[jswarren4]

The following changes:

1. Increase timeout for unlocking to one minute, up from 5 seconds
2. Use new "TryLock" functionality that will fail if another client has the lock instead of waiting to obtain the lock until the context times out.
3. Use one session to handle all locks instead of one session per lock as previously.
4. Implement local locker to preempt calls to etcd for locally held locks and because "TryLock" locks are reentrant.