Allow only 1 thread to send a closure to a given machine at the same time

This prevents a race where multiple threads see that machine X is
missing path P, and start sending it concurrently. Nix handles this
correctly, but it's still wasteful (especially for the case where P ==
GHC).

A more refined scheme would be to have per machine, per path locks.

src/hydra-queue-runner/hydra-queue-runner.cc

@@ -32,7 +32,6 @@ bool has(const C & c, const V & v)
 
 
 State::State()
-    : copyClosureTokenServer{maxParallelCopyClosure}
 {
     hydraData = getEnv("HYDRA_DATA");
     if (hydraData == "") throw Error("$HYDRA_DATA must be set");