[CF] Move InternalStats to ColumnFamilyData

Summary: InternalStats is a messy thing, keeping both DB data and column family data. However, it's better off living in ColumnFamilyData than in DBImpl. For now, at least.

Test Plan: make check

Reviewers: dhruba, kailiu, haobo, sdong

CC: leveldb

Differential Revision: https://reviews.facebook.net/D15879