This class is a GenericPatternCache subclass implementing a random
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
a randomly selected pattern in the cache.
PatternCacheRandom(int capacity,
PatternCompiler compiler)
Creates a PatternCacheRandom instance with a given cache capacity
and initialized to use a given PatternCompiler instance as a pattern
compiler.