[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/interpreter/wasm_loader.c

@@ -3237,6 +3237,10 @@ create_module(char *error_buf, uint32 error_buf_size)
 #endif
 #if WASM_ENABLE_DEBUG_INTERP != 0
     bh_list_init(&module->fast_opcode_list);
+    if (os_mutex_init(&module->ref_count_lock) != 0) {
+        wasm_runtime_free(module);
+        return NULL;
+    }
 #endif
     return module;
 }
@@ -3568,6 +3572,7 @@ wasm_loader_unload(WASMModule *module)
         wasm_runtime_free(fast_opcode);
         fast_opcode = next;
     }
+    os_mutex_destroy(&module->ref_count_lock);
 #endif
     wasm_runtime_free(module);
 }