Occurrence Indexer.
multi-thread fix. Keep sorted posting list.
multi-database for occurrence index.
Merge remote-tracking branch 'origin/huan'
support termFrequency and numDoc in DocOnly index.
tf idf for tokens
_totalTermFrequency fix.
construct compressed
Merge branch 'master' of https://github.com/owwlo/web_search_engine_course into huiyang

erge with master
token-based tf&idf for compressed index
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