Convert Locker to be instantiated.

As an instance, the Locker object is able to own the collection and its
poll duration configuration.  Calling code can then pass around the
Locker without receiving code being knowledgeable of the underlying
configuration.

Static methods in PHP are essentially just namespaced global methods -
there is very little different between Locker::readLock() and
Locker\readLock().  These methods are more difficult to use: they can't
be passed around easily, they require the shared parameters ($collection
and $pollDuration) in this case) to be kept and available for passing
everywhere the method needs to be called, and are more frustrating to
mock in tests if they can be mocked at all.