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/common/wasm_exec_env.h

@@ -98,6 +98,8 @@ typedef struct WASMExecEnv {
     /* used to support debugger */
     korp_mutex wait_lock;
     korp_cond wait_cond;
+    /* the count of threads which are joining current thread */
+    uint32 wait_count;
 #endif
 
 #if WASM_ENABLE_DEBUG_INTERP != 0