[source debug] refine some code in source debugging (#856)

- move the wait_cond from exec_env to debug_instance, so the debug thread can be waken up by any threads
- process more general query message from debugger
- refine debug instance create/destroy mechanism
- avoid creating debug instance during module instantiating
- avoid blocking execution thread during creating debug instance
- update related documents

core/iwasm/libraries/debug-engine/gdbserver.h

@@ -33,7 +33,10 @@ typedef struct WASMGDBServer {
 } WASMGDBServer;
 
 WASMGDBServer *
-wasm_launch_gdbserver(char *addr, int port);
+wasm_create_gdbserver(char *addr, int *port);
+
+bool
+wasm_gdbserver_listen(WASMGDBServer *server);
 
 void
 wasm_close_gdbserver(WASMGDBServer *server);