bdr: Overhauld of bdr_replicate_ddl_command

bdr: Overhauld of bdr_replicate_ddl_command

* execute the ddl on local node
* force the input query to be fully qualified
* use command filter to disallow dangerous DDL commands even when using
  bdr_replicate_ddl_command unless overriden by the GUC
* make it work in BDR and make sure it does not do double replication of
  the DDL
* rewrite of queue_dropped_objects into C
* rename the drop trigger so that the order of execution is correct
* recognize bdr node by replication slot with bdr output plugin