Update RestrictionTracker within Python to not be required to be thread safe

The commit https://github.com/apache/beam/commit/8faffb2bcf28ccab5e9a95322743cc60df65077c#diff-ed95abb6bc30a9ed07faef5c3fea93f0 modified the Java SDK removed the need for users to be thread safe and instead made the framework provide the necessary locking around a restriction tracker.