purge database before loading schema, take 2

This brings back the behavior introduced by #15394 which was reverted with 5c87b5c

The original patch introduced regressions where the wrong database was purged before loading the schema. The rake tasks no longer open specific connections but tell `DatabaseTasks` on what `configuration` to act on.

I had to create a temporary method `load_schema_for` because `DatabaseTasks` is public API and `load_schema` needs a new first argument. After the deprecation cycle we can rename `load_schema_for` back to `load_schema`.