Ignore missing column families

Summary:
Before this diff, whenever we Write to non-existing column family, Write() would fail.

This diff adds an option to not fail a Write() when WriteBatch points to non-existing column family. MongoDB said this would be useful for them, since they might have a transaction updating an index that was dropped by another thread. This way, they don't have to worry about checking if all indexes are alive on every write. They don't care if they lose writes to dropped index.

Test Plan: added a small unit test

Reviewers: sdong, yhchiang, ljin

Reviewed By: ljin

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D22143