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/state.hh

@@ -346,6 +346,9 @@ private:
 
     std::atomic<time_t> lastDispatcherCheck{0};
 
+    /* Destination store. */
+    std::shared_ptr<nix::Store> _destStore;
+
 public:
     State();