There are two main things.
First, remove the marker index caching. Thins makes it possible to
safely use function from a threaded environment.
Second, replace linear search with binary search, which speeds up
random lookup.
There is no measurable difference in the stabilization which had a
comment about caching nature of track lookup. The random lookup
complexity changed from O(N) to O(log N). In practice this also
unlikely to be measurable, but thread-safety worth it.
