Enable substitution on the build machines

Dependencies

• [2] 6EO3HVNA Merge remote-tracking branch 'origin/master' into binary-cache
• [3] LE4VZIY5 More stats
• [4] DWFTK56E Keep track of how many threads are waiting
• [5] CNLNT3T4 Allow only 1 thread to send a closure to a given machine at the same time
• [6] FITVNQ2S Keep track of the time we spend copying to/from build machines
• [*] 5AIYUMTB Basic remote building

Change contents

• replacement in src/hydra-queue-runner/build-remote.cc at line 241
  [3.56]→[2.0:63](∅→∅)
  − copyClosureTo(destStore, from, to, inputs, bytesSent);
  [3.56]

  [3.57]
  + copyClosureTo(destStore, from, to, inputs, bytesSent, true);