Get rid of mapDB for Occurrence
Merge remote-tracking branch 'origin/huiyang'
Add numDoc and termFrequency for occurrence index; Get rid of mapDB from doconly
commit for latter edit.
Totally get rid of mapDB
next doc
Fix read for _totalTermFrequency and _numDocs
fix stemmer calling issue
add cache for getFromIvtMap function
decompress postinglist