This class is a GenericPatternCache subclass implementing an LRU
(Least Recently Used) cache replacement policy. In other words,
patterns are added to the cache until it becomes full. Once the
cache is full, when a new pattern is added to the cache, it replaces
the least recently used pattern currently in the cache. This is probably
the best general purpose pattern cache replacement policy.
PatternCacheLRU(int capacity,
PatternCompiler compiler)
Creates a PatternCacheLRU instance with a given cache capacity,
and initialized to use a given PatternCompiler instance as a pattern
compiler.