More efficient Joinless

I've found the star instance prevalence filter is useless in most case, so I omit this
step in code. And there are another reason for doing this, the star instance is far
more frequent and space-consuming than clique instance, so I tend to generate clique
instance directlly when getting the star instance. That is, I don't need to store the
star instance, which can save much space.

However, the collection in Java is far from memory-friendly. No matter how I optimize my code, the data amount I can handle is limited. So I intend to include database storing the data, and I suppose the Berkeley DB is really suitbale for my application.
Merge branch 'master' of https://github.com/coder-chenzhi/ColocationPatternMinning