Add 'download-buffer-size' setting

We are piping curl downloads into `unpackTarfileToSink()`, but the
latter is typically slower than the former if you're on a fast
connection. So the download could appear unnecessarily slow. (There is
even a risk that if the Git import is *really* slow for whatever
reason, the TCP connection could time out.)

So let's make the download buffer bigger by default - 64 MiB is big
enough for the Nixpkgs tarball. Perhaps in the future, we could have
an unlimited buffer that spills data to disk beyond a certain
threshold, but that's probably overkill.

src/libstore/filetransfer.cc

@@ -858,7 +858,7 @@ void FileTransfer::download(
            buffer). We don't wait forever to prevent stalling the
            download thread. (Hopefully sleeping will throttle the
            sender.) */
-        if (state->data.size() > 1024 * 1024) {
+        if (state->data.size() > fileTransferSettings.downloadBufferSize) {
             debug("download buffer is full; going to sleep");
             state.wait_for(state->request, std::chrono::seconds(10));
         }