s4:dsdb/repl: make sure the working_schema prefix map is populated with the remote...

s4:dsdb/repl: make sure the working_schema prefix map is populated with the remote prefix map

We should create the working_schema prefix map before we try to
resolve the schema. This allows getting the same mapping (if there's not already
a conflict) and allows us to remove the implicit prefix mapping creation
in the prefix mapping lookup functions.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=12128

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>