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.cc

@@ -2,10 +2,10 @@
 
 namespace nix {
 
-LocalBinaryCacheStore::LocalBinaryCacheStore(ref<Store> localStore,
+LocalBinaryCacheStore::LocalBinaryCacheStore(const StoreFactory & storeFactory,
     const Path & secretKeyFile, const Path & publicKeyFile,
     const Path & binaryCacheDir)
-    : BinaryCacheStore(localStore, secretKeyFile, publicKeyFile)
+    : BinaryCacheStore(storeFactory, secretKeyFile, publicKeyFile)
     , binaryCacheDir(binaryCacheDir)
 {
 }