> (you don't have to strictly follow this form)

**Use case**

Sometimes, for really sophisticated scenarios it can be hard to control the amount of in-progress backups run externally. For the purpose of simpler control we can introduce server-level setting which disallows running concurrent backups of any type. 

Something like this in `config.xml` (enabled by default):
`<allow_concurrent_backups>true</allow_concurrent_backups>`

**Describe the solution you'd like**

It's important that backups can be executed in distributed way on multiple servers. Need to take it into account.

**Describe alternatives you've considered**
External check of `system.backups` table, but it can be complex.

{
    for (size_t i = 0; i != cluster_host_ids.size(); ++i)
    {
        for (size_t j = 0; j != cluster_host_ids[i].size(); ++j)
            if (cluster_host_ids[i][j] == host_id)
                return {i + 1, j + 1};
    }
    throw Exception(ErrorCodes::WRONG_BACKUP_SETTINGS, "Cannot determine shard number or replica number, the current host {} is not found in the cluster's hosts", host_id);
}
Strings BackupSettings::Util::filterHostIDs(const std::vector<Strings> & cluster_host_ids, size_t only_shard_num, size_t only_replica_num)