Fix thread manager issues (#962)

Fix the issue that joining a detached thread might result in joining hang,
resolve the issue by adding wait_count for a thread's exec_env to indicate
whether a thread needs to detach itself or not when it exits.

And add checks for the input exec_env for cluster's join/detach/cancel thread.

core/iwasm/libraries/thread-mgr/thread_manager.h

@@ -19,6 +19,7 @@ extern "C" {
 #if WASM_ENABLE_DEBUG_INTERP != 0
 typedef struct WASMDebugInstance WASMDebugInstance;
 #endif
+
 typedef struct WASMCluster {
     struct WASMCluster *next;