Simplifies cancel operations. #549

Job	Run time
B2 msvc-14.2	6m 43s
CMake gcc-11 17 Debug	7m 50s
B2 msvc-14.3	8m 43s
CMake msvc-14.2 Debug C++17	9m 33s
CMake gcc-11 20 Release	10m 27s
B2 gcc-11	5m 14s
CMake msvc-14.2 Release C++17	9m 23s
CMake clang-11 17 Debug	6m 18s
CMake msvc-14.3 Debug C++20	8m 49s
B2 clang-14	5m 34s
CMake clang-11 20 Debug	6m 22s
CMake msvc-14.3 Release C++20	10m 9s
CMake clang-13 17 Release	9m 29s
CMake clang-13 20 Release	9m 58s
CMake clang-14 17 Debug -stdlib=libc++	7m 14s
CMake clang-14 20 Release -stdlib=libc++	10m 53s
	2h 12m 39s

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