RemoteStore::addToStore(): Pass content-addressability assertion

... and use this in Downloader::downloadCached(). This fixes

  $ nix-build https://nixos.org/channels/nixos-16.09-small/nixexprs.tar.xz -A hello
  error: cannot import path ‘/nix/store/csfbp1s60dkgmk9f8g0zk0mwb7hzgabd-nixexprs.tar.xz’ because it lacks a valid signature

src/nix-daemon/nix-daemon.cc

@@ -582,6 +582,7 @@ static void performOp(ref<LocalStore> store, bool trusted, unsigned int clientVe
         info.narSize = readLongLong(from);
         info.ultimate = readLongLong(from);
         info.sigs = readStrings<StringSet>(from);
+        info.ca = readString(from);
         auto nar = make_ref<std::string>(readString(from));
         auto repair = readInt(from) ? true : false;
         auto dontCheckSigs = readInt(from) ? true : false;