> (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.

{
    Strings collected_host_ids;
    auto collect_replicas = [&](size_t shard_index)
    {
        const auto & shard = cluster_host_ids[shard_index - 1];
        if (only_replica_num)
        {
            if (only_replica_num <= shard.size())
                collected_host_ids.push_back(shard[only_replica_num - 1]);
        }
        else
        {
            for (size_t replica_index = 1; replica_index <= shard.size(); ++replica_index)
                collected_host_ids.push_back(shard[replica_index - 1]);
        }
    };
    if (only_shard_num)
    {
        if (only_shard_num <= cluster_host_ids.size())
            collect_replicas(only_shard_num);
    }
    else
    {
        for (size_t shard_index = 1; shard_index <= cluster_host_ids.size(); ++shard_index)
            collect_replicas(shard_index);
    }
    return collected_host_ids;
}
}