refactor: add internal Promise/Future classes for future use (#521) #2782

Job	Run time
contract-tests-curl	3m 25s
build-test	6m 40s
build-test-client-mac	4m 31s
build-test-client-windows	14m 47s
build-test-client-mac-curl	5m 8s
contract-tests	3m 17s
build-test-client-windows-curl	15m 41s
build-test-curl	6m 8s
	59m 37s

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