> (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.

{
    try
    {
        
        if (checkZooKeeperConfigIsLocal(getConfigRef(), "zookeeper"))
        {
            LOG_DEBUG(shared->log, "Keeper server is participant of the main zookeeper cluster, will try to connect to it");
            getZooKeeper();
            
            return true;
        }
        else
        {
            Poco::Util::AbstractConfiguration::Keys keys;
            getConfigRef().keys("auxiliary_zookeepers", keys);
            
            for (const auto & aux_zk_name : keys)
            {
                if (checkZooKeeperConfigIsLocal(getConfigRef(), "auxiliary_zookeepers." + aux_zk_name))
                {
                    LOG_DEBUG(shared->log, "Our Keeper server is participant of the auxiliary zookeeper cluster ({}), will try to connect to it", aux_zk_name);
                    getAuxiliaryZooKeeper(aux_zk_name);
                    
                    return true;
                }
            }
        }