Prevent attempts to create/drop/rename bdr_supervisordb

Prevent attempts to create/drop/rename  bdr_supervisordb

When the BDR extension is loaded, BDR its self can create
bdr_supervisordb, but applications cannot. This gives users
a better error message if they try to drop it, too, e.g.

DROP DATABASE bdr_supervisordb;
ERROR:  The BDR extension reserves the database name bdr_supervisordb
for its own use
HINT:  Use a different database name