[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/common/wasm_runtime_common.h

@@ -494,6 +494,12 @@ wasm_runtime_call_wasm_v(WASMExecEnv *exec_env,
                          uint32 num_results, wasm_val_t *results,
                          uint32 num_args, ...);
 
+#if WASM_ENABLE_DEBUG_INTERP != 0
+/* See wasm_export.h for description */
+WASM_RUNTIME_API_EXTERN uint32
+wasm_runtime_start_debug_instance(WASMExecEnv *exec_env);
+#endif
+
 /**
  * Call a function reference of a given WASM runtime instance with
  * arguments.