queue-runner: release machine reservation while copying outputs

This allows for better builder usage when the queue runner is busy. To
avoid running into uncontrollable imbalances between builder/queue
runner, we only release the machine reservation after the local
throttler has found a slot to start copying the outputs for that build.

As opposed to asserting uniqueness to understand resource utilization,
we just switch to using `std::unique_ptr`.

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

@@ -398,6 +398,7 @@ private:
 };
 
 void State::buildRemote(ref<Store> destStore,
+    std::unique_ptr<MachineReservation> reservation,
     ::Machine::ptr machine, Step::ptr step,
     const ServeProto::BuildOptions & buildOptions,
     RemoteResult & result, std::shared_ptr<ActiveStep> activeStep,
@@ -546,6 +547,14 @@ void State::buildRemote(ref<Store> destStore,
         }
         SemaphoreReleaser releaser(&localWorkThrottler);
 
+        /* Once we've started copying outputs, release the machine reservation
+         * so further builds can happen. We do not release the machine earlier
+         * to avoid situations where the queue runner is bottlenecked on
+         * copying outputs and we end up building too many things that we
+         * haven't been able to allow copy slots for. */
+        reservation.reset();
+        wakeDispatcher();
+
         StorePathSet outputs;
         for (auto & [_, realisation] : buildResult.builtOutputs)
             outputs.insert(realisation.outPath);