universal compaction picker: use double for potential overflow

Summary: There is a possible overflow case in universal compaction picker. Use double to make the logic straight-forward

Test Plan: make all check

Reviewers: yhchiang, igor, MarkCallaghan, ljin

Reviewed By: ljin

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D23817
Initial steps on providing column family support in RocksJava
Added JavaDoc for listColumnFamilies method.
Added throws clause to method listColumnFamilies.
Several changes:
- Enhanced JavaDoc
- Remove support added
- DropColumnFamily support added
- WriteBatch support added
- Put/Get completed