Vsencoding for positionlist

apply vs encoding for positionlist
I'm sorry I was a little confused when I wrote this comment. In fact this comment is totally wrong. I have corrected it. Actually I mean we use vsencoding for all positions lists, and discard interpolative encoding, so I delete the code about interpolative encoding. And here is a simple test. http://trac.xapian.org/wiki/GSoC2014/Posting%20list%20encoding%20improvements/Journal#June30