bdr.table_set_replication_sets should permit unsafe commands

bdr.table_set_replication_sets should permit unsafe commands

In 4005c1d bdr_replicate_ddl_command was changed not to override the
command filter, but this broke replication sets in UDR, since they have
to use SECURITY LABEL.

SET bdr.permit_unsafe_ddl_commands on bdr.table_set_replication_sets
since it knows what it's doing.

Also add regression test coverage of replication sets to UDR by
using bdr.bdr_replicate_ddl_command(...) to wrap DDL. This requires
explicitly schema-qualifying objects otherwise the error:

    "no schema has been selected to create in"

will arise.

Fixes #67