Implementation:
* Added server level settings to disallow concurrent backups and restores, which are read and set when BackupWorker is created in Context.
* Settings are set to true by default.
* Before starting backup or restores, added a check to see if any other backups/restores are running (except internal ones).
Testing:
* Added a test test_backup_and_restore_on_cluster/test_disallow_concurrency.