Detect and store BDR major version number when executing "is_bdr_db()"

BDR3 metadata structure is very different to BDR1/2, so we'll need to
generate queries according to version.

repmgr.h

@@ -49,6 +49,8 @@
 #define REPLICATION_TYPE_BDR	  2
 
 #define UNKNOWN_SERVER_VERSION_NUM -1
+#define UNKNOWN_BDR_VERSION_NUM -1
+
 #define UNKNOWN_TIMELINE_ID -1
 #define UNKNOWN_SYSTEM_IDENTIFIER 0