distinguish cmagent backend from noraml backend for dms

src/include/threadpool/threadpool_worker.h

@@ -55,6 +55,7 @@ typedef struct Backend {
     bool is_autovacuum; /* is it an autovacuum process? */
     volatile bool dead_end; /* is it going to send an quit? */
     volatile int flag;
+    int backend_type;
     Dlelem elem; /* list link in BackendList */
 } Backend;