Worker: Fix sending callback leaks #4567

Job	Run time
ci (ubuntu-20.04, gcc, g++, 20, Release)	7m 50s
ci (ubuntu-20.04, gcc, g++, 20, Debug)	8m 42s
ci (ubuntu-20.04, clang, clang++, 20, Release)	8m 2s
ci (ubuntu-20.04, clang, clang++, 20, Debug)	7m 45s
ci (ubuntu-22.04, gcc, g++, 20, Release)	6m 59s
ci (ubuntu-22.04, gcc, g++, 20, Debug)	6m 15s
ci (ubuntu-22.04, clang, clang++, 20, Release)	8m 55s
ci (ubuntu-22.04, clang, clang++, 20, Debug)	7m 47s
ci (macos-12, gcc, g++, 20, Release)	12m 50s
ci (macos-12, gcc, g++, 20, Debug)	5m 44s
ci (macos-14, clang, clang++, 20, Release)	4m 1s
ci (macos-14, clang, clang++, 20, Debug)	3m 43s
ci (windows-2022, cl, cl, 20, Release)	7m 24s
ci (windows-2022, cl, cl, 20, Debug)	5m 49s
	1h 41m 46s

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