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/state.hh

@@ -12,7 +12,6 @@
 #include "pathlocks.hh"
 #include "pool.hh"
 #include "sync.hh"
-#include "token-server.hh"
 
 #include "store-api.hh"
 #include "derivations.hh"
@@ -136,6 +135,10 @@ struct Machine
         counter nrStepsDone{0};
         counter totalStepTime{0}; // total time for steps, including closure copying
         counter totalStepBuildTime{0}; // total build time for steps
+
+        /* Mutex to prevent multiple threads from sending data to the
+           same machine (which would be inefficient). */
+        std::mutex sendLock;
     };
 
     State::ptr state;
@@ -191,10 +194,6 @@ private:
     nix::Path machinesFile;
     struct stat machinesFileStat;
 
-    /* Token server limiting the number of threads copying closures in
-       parallel to prevent excessive I/O load. */
-    TokenServer copyClosureTokenServer;
-
     /* Various stats. */
     time_t startedAt;
     counter nrBuildsRead{0};