libstore: make baseRetryTimeMs a FileTransfer property

we don't even need this outside of tests. maybe we should not do
automatic retries at this level at all and use retrying wrappers
instead? at some point we may have to do this, but not just yet.

Change-Id: If0088aa55215be81f1770c25b3bb1b5268c65cf8

src/libstore/filetransfer.hh

@@ -60,7 +60,6 @@ struct FileTransferRequest
     bool verifyTLS = true;
     bool head = false;
     size_t tries = fileTransferSettings.tries;
-    unsigned int baseRetryTimeMs = 250;
 
     FileTransferRequest(std::string_view uri)
         : uri(uri) { }
@@ -123,7 +122,7 @@ ref<FileTransfer> getFileTransfer();
  *
  * Prefer getFileTransfer() to this; see its docs for why.
  */
-ref<FileTransfer> makeFileTransfer();
+ref<FileTransfer> makeFileTransfer(std::optional<unsigned int> baseRetryTimeMs = {});
 
 class FileTransferError : public Error
 {