Move functions from VersionSet to Version

Summary:
There were some functions in VersionSet that had no reason to be there instead of Version. Moving them to Version will make column families implementation easier.

The functions moved are:
* NumLevelBytes
* LevelSummary
* LevelFileSummary
* MaxNextLevelOverlappingBytes
* AddLiveFiles (previously AddLiveFilesCurrentVersion())
* NeedSlowdownForNumLevel0Files

The diff continues on (and depends on) D15171

Test Plan: make check

Reviewers: dhruba, haobo, kailiu, sdong, emayanke

Reviewed By: sdong

CC: leveldb

Differential Revision: https://reviews.facebook.net/D15183