Mv sequential tuning

The branch contains four independent changes, but all related to the customer issue:

- Create alternate throttle value for single vnode usage
- Restore the IsTrivialMove() function so that some compactions are simply renames
- Correct Options::mmap_size usage so that write_buffer is not forced to 20Mbytes all the time
- Implement concept of mv-compress-bypass branch in current code to eliminate compression attempts known to fail

Detail description: https://github.com/basho/leveldb/wiki/mv-sequential-tuning