VersionSet cleanup

Summary:
Removed icmp_ from VersionSet (since it's per-column-family, not per-DB-instance)
Unfriended VersionSet and ColumnFamilyData (yay!)
Removed VersionSet::NumberLevels()
Cleaned up DBImpl

Test Plan: make check

Reviewers: dhruba, haobo, kailiu

CC: leveldb

Differential Revision: https://reviews.facebook.net/D15819