Package locker provides a mechanism for creating finer-grained locking to help
free up more global locks to handle other tasks.
The implementation looks close to a sync.Mutex, however the user must provide a
reference to use to refer to the underlying lock when locking and unlocking,
and unlock may generate an error.
If a lock with a given name does not exist when `Lock` is called, one is
created.
Lock references are automatically cleaned up on `Unlock` if nothing else is
waiting for the lock.