> (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 (coordination)
                coordination->setError(current_host, exception);
        }
        catch (...)
        {
        }
    }
    
    template <typename CoordinationType>
    void sendCurrentExceptionToCoordination(std::shared_ptr<CoordinationType> coordination, const String & current_host)
    {
        try
        {
            throw;
        }
        catch (const Exception & e)
        {
            sendExceptionToCoordination(coordination, current_host, e);
        }
        catch (...)
        {
            if (coordination)
                coordination->setError(current_host, Exception{getCurrentExceptionCode(), getCurrentExceptionMessage(true, true)});
        }
    }
    bool isFinalStatus(BackupStatus status)