gitaly-backup: Add parallel flag to partition create

In #6272 (closed) we created partition backup command for gitaly-backup, but we implemented it without any concurrency. Existing pipeline logic is heavily rely on repositories while we don't have any repository information for partition backups.

We also don't need as complicated concurrency control since we have control over the entire backup process.In this MR I introduced concurrency using goroutines.

Closes #6414 (closed)