Index name uniqueness validation in CFMetaData is not entirely correct

The check in CFMetaData.validate() relies on external global state (Schema.instance to fetch all index names in the keyspace. However, in many cases the validation will be performed without all table instances registered in Schema.isntance yet. The check should live in KeyspaceMetadata instead, where no such access is required.

Things broken by it right now: multiple tests, and Thrift's system_add_keyspace, at least.