StratumServer: fixed memory leak on shutdown #1669

Job	Run time
build-alpine-static (x86_64, -flto=2 -fuse-linker-plugin -ffunction-sections -Wno-error=inline)	4m 12s
build-freebsd (freebsd, x86-64, 12.4, ubuntu-22.04)	54m 17s
build-alpine-static (aarch64, -flto=2 -fuse-linker-plugin -ffunction-sections -Wno-error=inline -...	41m 18s
build-macos	7m 12s
build-macos-aarch64	3m 2s
build-ubuntu (ubuntu-20.04, gcc-11, g++-11)	3m 27s
build-ubuntu-aarch64 (ubuntu-20.04, -flto=2 -fuse-linker-plugin -ffunction-sections -mfix-cortex-...	8m 19s
build-ubuntu (ubuntu-22.04, gcc-12, g++-12)	4m 54s
build-ubuntu-aarch64 (ubuntu-22.04, -flto=2 -fuse-linker-plugin -ffunction-sections -mfix-cortex-...	7m 40s
build-ubuntu-static-libs (-flto=2 -fuse-linker-plugin -ffunction-sections -Wno-error=inline)	5m 53s
build-windows-msbuild (Visual Studio 16 2019, 2019, C:\Program Files (x86)\Microsoft Visual Studi...	9m 32s
build-windows-msbuild (Visual Studio 16 2019, 2019, C:\Program Files (x86)\Microsoft Visual Studi...	7m 51s
build-windows-msbuild (Visual Studio 16 2019, 2019, C:\Program Files (x86)\Microsoft Visual Studi...	9m 16s
build-windows-msys2 (gcc, g++, -flto=2 -fuse-linker-plugin -ffunction-sections -Wno-error=maybe-u...	14m 33s
build-windows-msys2 (clang, clang++, -flto -ffunction-sections -fuse-ld=lld -Wno-unused-command-l...	11m 53s
build-openbsd (openbsd, x86-64, 7.2, ubuntu-22.04)	55m 36s
	4h 8m 55s

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