Summary:
An entry of ConstantColumnFamilyInfo is created when:
1. DB::Open
2. CreateColumnFamily.

However, there are cases that DB::Open could also call CreateColumnFamily
when create_missing_column_families=true.  As a result, it will create
duplicate ConstantColumnFamilyInfo and one of them would be leaked.

Test Plan: ./deletefile_test

Reviewers: igor, sdong, ljin

Reviewed By: ljin

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D29307