MINIFICPP-2771 Use separate state managers in schedule and trigger threads

[lordgamez]

https://issues.apache.org/jira/browse/MINIFICPP-2771

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[szaszm]

In parameters, I'd use references instead of not_null pointers, but in data members, I'd keep the not_null pointer, because members generally shouldn't be const.

Suggested change

	explicit ListingStateManager(gsl::not_null<core::StateManager*> state_manager)
	: state_manager_(state_manager) {
	explicit ListingStateManager(core::StateManager& state_manager)
	: state_manager_(&state_manager) {

[szaszm]

Shouldn't this be an error, like an exception?

[szaszm]

Suggested change

	void S3Wrapper::initializeMultipartUploadStateStorage(gsl::not_null<core::StateManager*> state_manager) {
	multipart_upload_storage_ = std::make_unique<MultipartUploadStateStorage>(state_manager);
	void S3Wrapper::initializeMultipartUploadStateStorage(core::StateManager& state_manager) {
	multipart_upload_storage_ = std::make_unique<MultipartUploadStateStorage>(state_manager);