Need more data in memtable to flush due to 11da8b
CF cleanup part 2
[CF] WriteBatch to take in ColumnFamilyHandle

Summary: Client doesn't need to know anything about ColumnFamily ID. By making WriteBatch take ColumnFamilyHandle as a parameter, we can eliminate method GetID() from ColumnFamilyHandle

Test Plan: column_family_test

Reviewers: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D16887