[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/lib-pthread/lib_pthread_wrapper.c

@@ -332,7 +332,7 @@ create_cluster_info(WASMCluster *cluster)
     if (!(node = wasm_runtime_malloc(sizeof(ClusterInfoNode)))) {
         return NULL;
     }
-    memset(node, 0, sizeof(WASMCluster));
+    memset(node, 0, sizeof(ClusterInfoNode));
 
     node->thread_list = &node->thread_list_head;
     ret = bh_list_init(node->thread_list);