Clean up code for BenchHub

[at15]

Major issues

• benchmark related code will be split into libstdb-go and xephon-b
• #62 move collector somewhere else, might be benchhub, or gommon
• add tracing on server side to know where is our hot spot
• use go.ice as server framework

TODO

• [x] group and clean up old issues, a new tag pre-benchhub should be added
• [ ] make the cassandra backend work again, it should be very easy without rich tag support
• [ ] recap the column store, it should support read at least
  • [ ] the inverted index can just be kept in memory and rebuilt when start, I don't think there is time to deal with high carnality
  • might take the time and port it to gegecece ...