delete: make impl::refs atomic #593

Job	Run time
Test with C++20 modules enabled / test	11m 19s
Test with DPDK enabled / test	22m 20s
Test (clang++, C++20, debug) / test	15m 54s
Test (clang++, C++20, dev) / test	16m 13s
Test (clang++, C++20, release) / test	15m 41s
Test (clang++, C++23, debug) / test	16m 23s
Test (clang++, C++23, dev) / test	17m 6s
Test (clang++, C++23, release) / test	16m 1s
Test (g++, C++20, debug) / test	19m 33s
Test (g++, C++20, dev) / test	11m 27s
Test (g++, C++20, release) / test	15m 42s
Test (g++, C++23, debug) / test	18m 11s
Test (g++, C++23, dev) / test	12m 30s
Test (g++, C++23, release) / test	16m 18s
	3h 44m 38s

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