nixd/Server: split worker in a dedicated class #1374

Job	Run time
nix-build-develop-clang (none, release, true)	4m 11s
nix-build-develop-clang (none, release, false)	4m 47s
nix-build-develop-clang (none, debug, true)	3m 47s
nix-build-develop-clang (none, debug, false)	4m 4s
nix-build-develop-gcc (none, release, true)	5m 29s
nix-build-develop-gcc (none, release, false)	6m 4s
nix-build-develop-gcc (none, debug, true)	4m 37s
nix-build-develop-gcc (none, debug, false)	4m 15s
nix-build-develop-gcc (undefined, release, true)	13m 4s
nix-build-develop-gcc (undefined, release, false)	9m 30s
nix-build-develop-gcc (undefined, debug, true)	5m 12s
nix-build-develop-gcc (undefined, debug, false)	7m 24s
	1h 12m 24s

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