Use a single BinaryCacheStore for all threads

This will make it easier to do caching / keep stats. Also, we won't
have S3Client's connection pooling if we create multiple S3Client
instances.

src/hydra-queue-runner/local-binary-cache-store.hh

@@ -12,7 +12,7 @@ private:
 
 public:
 
-    LocalBinaryCacheStore(ref<Store> localStore,
+    LocalBinaryCacheStore(const StoreFactory & storeFactory,
         const Path & secretKeyFile, const Path & publicKeyFile,
         const Path & binaryCacheDir);