LUCENE-3069 Lucene should have an entirely memory resident term dictionary

by Han Jiang for Apache Software Foundation

I aim to improve codec performance by implementing a memory resident term dictionary. The proposed work includes generalization/refactoring of the backend design of PostingsConsumer/Producer, implementation of a pluggable and memory resident term dictionary module, and optimization. I'll also try to improve related codes on oal.util.FST, and introduce a real data set to further experiments.