nixd/Server: use #included ReplyRAII #1397

Job	Run time
nix-build-develop-clang (none, release, true)	5m 45s
nix-build-develop-clang (none, release, false)	4m 2s
nix-build-develop-clang (none, debug, true)	5m 10s
nix-build-develop-clang (none, debug, false)	3m 41s
nix-build-develop-gcc (none, release, true)	5m 22s
nix-build-develop-gcc (none, release, false)	5m 26s
nix-build-develop-gcc (none, debug, true)	5m 47s
nix-build-develop-gcc (none, debug, false)	4m 42s
nix-build-develop-gcc (undefined, release, true)	14m 5s
nix-build-develop-gcc (undefined, release, false)	11m 28s
nix-build-develop-gcc (undefined, debug, true)	6m 48s
nix-build-develop-gcc (undefined, debug, false)	5m 29s
	1h 17m 45s

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