ColumnFamilySet

Summary:
I created a separate class ColumnFamilySet to keep track of column families. Before we did this in VersionSet and I believe this approach is cleaner.

Let me know if you have any comments. I will commit tomorrow.

Test Plan: make check

Reviewers: dhruba, haobo, kailiu, sdong

CC: leveldb

Differential Revision: https://reviews.facebook.net/D15357