WriteBatchWithIndex to allow different Comparators for different column families

Summary:
Previously, one single column family is given to WriteBatchWithIndex to index keys for all column families. An extra map from column family ID to comparator is maintained which can override the default comparator given in the constructor. A WriteBatchWithIndex::SetComparatorForCF() is added for user to add comparators per column family.

Also move more codes into anonymous namespace.

Test Plan: Add a unit test

Reviewers: ljin, igor

Reviewed By: igor

Subscribers: dhruba, leveldb, yhchiang

Differential Revision: https://reviews.facebook.net/D23355