Destroy child thread's exec_env before destroying its module instance and
add the process into cluster's lock to avoid possible data race: if exec_env
is removed from custer's exec_env_list and destroyed later, the main thread
may not wait it and start to destroy the wasm runtime, and the destroying
of the sub thread's exec_env may free or overread/written an destroyed or
re-initialized resource.
And fix an issue in wasm_cluster_cancel_thread.
e516de8ec7