Simplifies async operations. #547

Job	Run time
B2 msvc-14.2	7m 35s
CMake gcc-11 17 Debug	8m 37s
B2 msvc-14.3	8m 37s
B2 gcc-11	5m 7s
CMake gcc-11 20 Release	10m 31s
CMake msvc-14.2 Debug C++17	8m 27s
B2 clang-14	5m 31s
CMake clang-11 17 Debug	5m 59s
CMake msvc-14.2 Release C++17	6m 54s
CMake clang-11 20 Debug	6m 24s
CMake msvc-14.3 Debug C++20	6m 59s
CMake clang-13 17 Release	9m 24s
CMake msvc-14.3 Release C++20	8m 51s
CMake clang-13 20 Release	9m 43s
CMake clang-14 17 Debug -stdlib=libc++	7m 12s
CMake clang-14 20 Release -stdlib=libc++	11m 5s
	2h 6m 56s

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