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.
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