Fix persistent hash mismatch errors when importing

Dependencies

• [2] N3G7LLGE Get data needed by getBuildOutput() from the incoming NAR in a streaming fashion
• [3] AOQVBDWP Copy paths in the right order
• [4] 5JMO43B4 buildRemote(): Copy paths to the destination store in O(1) memory
• [*] 5AIYUMTB Basic remote building

Change contents

• edit in src/hydra-queue-runner/build-remote.cc at line 484
  [3.1896]→[3.1896:2000](∅→∅)
  − to << cmdDumpStorePath << localStore->printStorePath(path);

  − to.flush();
• edit in src/hydra-queue-runner/build-remote.cc at line 490
  [2.491]

  [2.491]
  + /* Note: we should only send the command to dump the store

  + path to the remote if the NAR is actually going to get read

  + by the destination store, which won't happen if this path

  + is already valid on the destination store. Since this

  + lambda function only gets executed if someone tries to read

  + from source2, we will send the command from here rather

  + than outside the lambda. */

  + to << cmdDumpStorePath << localStore->printStorePath(path);

  + to.flush();

  +