Add configure option --with-bdr-only

Builds repmgr with only BDR functionality; other code is disabled
at critical points.

repmgrd.c

@@ -362,12 +362,18 @@ start_monitoring(void)
 	{
 		switch (local_node_info.type)
 		{
+#ifndef BDR_ONLY
 			case PRIMARY:
 				monitor_streaming_primary();
 				break;
 			case STANDBY:
 				monitor_streaming_standby();
 				break;
+#else
+			case PRIMARY:
+			case STANDBY:
+				return;
+#endif
 			case BDR:
 				monitor_bdr();
 				return;