212 async exec never returns when the network is disconnected and then restored #533

Job	Run time
B2 msvc-14.2	7m 9s
B2 gcc-11	4m 53s
B2 msvc-14.3	8m 48s
CMake msvc-14.2 Debug C++17	6m 16s
B2 clang-14	5m 12s
CMake gcc-11 17 Debug	8m 59s
CMake msvc-14.2 Release C++17	6m 28s
CMake msvc-14.3 Debug C++20	9m 24s
CMake gcc-11 20 Release	11m 37s
CMake msvc-14.3 Release C++20	9m 14s
CMake clang-11 17 Debug	6m 19s
CMake clang-11 20 Debug	6m 47s
CMake clang-13 17 Release	9m 29s
CMake clang-13 20 Release	10m 5s
CMake clang-14 17 Debug -stdlib=libc++	8m 32s
CMake clang-14 20 Release -stdlib=libc++	12m 5s
	2h 11m 17s

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