use talloc with the global schema consistently

use talloc with the global schema consistently

Before this change, the first opener of the sam ldb context would
become the owner of the global schema, then the autofree context got a
reference to the schema. Any subsequent opens of the sam ldb also got
a reference. This meant that the talloc hierarchy was inconsistent
between the first sam ldb open and subsequent opens. With this change
the autofree context becomes the owner of the global schema, and all
ldb contexts get a reference.