Method addCallback

flow/include/flow/ThreadHelper.actor.h:181–188

	ThreadMultiCallback() {}

	ThreadCallback* addCallback(ThreadCallback* callback) override {
		// May be triggered by a waitForAll on a vector with the same future in it more than once
		UNSTOPPABLE_ASSERT(callback->getHolder(this) == nullptr);

		callback->addHolder(this, callbacks.size());
		callbacks.push_back(callback);
		return (ThreadCallback*)this;
	}

	void clearCallback(ThreadCallback* callback) override {
		MultiCallbackHolder* h = callback->getHolder(this);