Logical replication

- Add PUBLICATION catalogs and DDL
- Add SUBSCRIPTION catalog and DDL
- Define logical replication protocol and output plugin
- Add logical replication workers

From: Petr Jelinek <petr@2ndquadrant.com>
Reviewed-by: Steve Singer <steve@ssinger.info>
Reviewed-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Erik Rijkers <er@xs4all.nl>
Reviewed-by: Peter Eisentraut <peter.eisentraut@2ndquadrant.com>

src/backend/commands/dropcmds.c

@@ -441,6 +441,10 @@ does_not_exist_skipping(ObjectType objtype, List *objname, List *objargs)
 				}
 			}
 			break;
+		case OBJECT_PUBLICATION:
+			msg = gettext_noop("publication \"%s\" does not exist, skipping");
+			name = NameListToString(objname);
+			break;
 		default:
 			elog(ERROR, "unrecognized object type: %d", (int) objtype);
 			break;