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/s3-binary-cache-store.hh

@@ -18,7 +18,7 @@ private:
 
 public:
 
-    S3BinaryCacheStore(ref<Store> localStore,
+    S3BinaryCacheStore(const StoreFactory & storeFactory,
         const Path & secretKeyFile, const Path & publicKeyFile,
         const std::string & bucketName);