[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/include/wasm_export.h

@@ -408,6 +408,26 @@ wasm_runtime_create_exec_env(wasm_module_inst_t module_inst,
 WASM_RUNTIME_API_EXTERN void
 wasm_runtime_destroy_exec_env(wasm_exec_env_t exec_env);
 
+/**
+ * Start debug instance based on given execution environment.
+ * Note:
+ *   The debug instance will be destroyed during destroying the
+ *   execution environment, developers don't need to destroy it
+ *   manually.
+ *   If the cluster of this execution environment has already
+ *   been bound to a debug instance, this function will return true
+ *   directly.
+ *   If developer spawns some exec_env by wasm_runtime_spawn_exec_env,
+ *   don't need to call this function for every spawned exec_env as
+ *   they are sharing the same cluster with the main exec_env.
+ *
+ * @param exec_env the execution environment to start debug instance
+ *
+ * @return debug port if success, 0 otherwise.
+ */
+WASM_RUNTIME_API_EXTERN uint32_t
+wasm_runtime_start_debug_instance(wasm_exec_env_t exec_env);
+
 /**
  * Initialize thread environment.
  * Note: