[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/debug_engine.h

@@ -42,6 +42,8 @@ typedef struct WASMDebugInstance {
     WASMCluster *cluster;
     uint32 id;
     korp_tid current_tid;
+    korp_mutex wait_lock;
+    korp_cond wait_cond;
 } WASMDebugInstance;
 
 typedef enum WASMDebugEventKind {
@@ -160,6 +162,9 @@ uint64
 wasm_debug_instance_wait_thread(WASMDebugInstance *instance, uint64 tid,
                                 uint32 *status);
 
+uint32
+wasm_debug_instance_get_thread_status(WASMDebugInstance *instance, uint64 tid);
+
 bool
 wasm_debug_instance_singlestep(WASMDebugInstance *instance, uint64 tid);