Option for pt-osc to limit foreign key search to table's schema

Currently pt-online-schema-change allows for foreign keys to exist in other schemas. A very common use case is many schemas which share a server but have no foreign key references across schemas. In these cases, pt-online-schema-change can be slow to query information_schema for all references to the table being altered. It would be helpful to optionally short circuit the search for foreign keys to just the schema of the table being altered.