Column family support for DB::OpenForReadOnly()

Summary: When opening DB in read-only mode, client can choose to only specify a subset of column families ("default" column family can't be omitted, though)

Test Plan: added a unit test in column_family_test

Reviewers: haobo, sdong, ljin, dhruba

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D17565