Uses asio::deferred_t as default completion type. #562

Job	Run time
B2 gcc-11	5m 10s
B2 msvc-14.2	8m 17s
CMake gcc-11 17 Debug	7m 25s
B2 clang-14	5m 32s
B2 msvc-14.3	7m 59s
CMake msvc-14.2 Debug C++17	6m 57s
CMake gcc-11 20 Release	9m 53s
CMake msvc-14.2 Release C++17	5m 55s
CMake clang-11 17 Debug	6m 12s
CMake clang-11 20 Debug	6m 29s
CMake msvc-14.3 Debug C++20	7m 46s
CMake clang-13 17 Release	9m 13s
CMake msvc-14.3 Release C++20	7m 57s
CMake clang-13 20 Release	9m 42s
CMake clang-14 17 Debug -stdlib=libc++	7m 15s
CMake clang-14 20 Release -stdlib=libc++	10m 34s
	2h 2m 16s

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