Summary:
In some cases, we don't have to get really accurate number. Something like 10% off is fine, we can create a new option for that use case. In this case, we can calculate size for full files first, and avoid estimation inside SST files if full files got us a huge number. For example, if we already covered 100GB of data, we should be able to skip partial dives into 10 SST files of 30MB.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5609

Differential Revision: D16433481

Pulled By: elipoz

fbshipit-source-id: 5830b31e1c656d0fd3a00d7fd2678ddc8f6e601b