buildRemote(): Support arbitrary stores

Rather than re-implementing the legacy SSH store protocol, we now use
Store directly. To be able to support build cancellation, the actual
build is performed by a helper process (hydra-build-step), which can
be killed by hydra-queue-runner if the corresponding step is
cancelled.

src/hydra-queue-runner/state.hh

@@ -359,9 +359,10 @@ private:
     counter nrStepsStarted{0};
     counter nrStepsDone{0};
     counter nrStepsBuilding{0};
+    #if 0
     counter nrStepsCopyingTo{0};
     counter nrStepsCopyingFrom{0};
-    counter nrStepsWaiting{0};
+    #endif
     counter nrUnsupportedSteps{0};
     counter nrRetries{0};
     counter maxNrRetries{0};
@@ -370,8 +371,6 @@ private:
     counter nrQueueWakeups{0};
     counter nrDispatcherWakeups{0};
     counter dispatchTimeMs{0};
-    counter bytesSent{0};
-    counter bytesReceived{0};
     counter nrActiveDbUpdates{0};
 
     /* Specific build to do for --build-one (testing only). */