[k2] fix exception handling in wait_multi and some other minor fixes … #2516

Job	Run time
focal / g++-10 / c++20 / asan=on / ubsan=off	33m 37s
jammy / g++ / c++20 / asan=on / ubsan=off	30m 54s
focal / clang++ / c++17 / asan=off / ubsan=on	32m 9s
	1h 36m 40s

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