Fix race condition, when methods `RedisLockRegistry#expireUnusedOlderThan` and `RedisLockRegistry#obtain` are executed successively.
It's possible to delete the lock from `RedisLockRegistry#expireUnusedOlderThan` method, when lock is created but is not acquired (`RedisLock#getLockedAt = 0`)
It can lead to the situation, when `RedisLockRegistry#obtain` returns multiple locks with the same redis-key, which shouldn't happen at all.
* Skip locks from expiration when their `lockedAt == 0` - new, not acquired yet.
**Cherry-pick to `6.0.x` & `5.5.x`**
d7150685b1