> (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.

{
        if (!coordination_zk_path.empty())
        {
            auto get_zookeeper = [global_context = context->getGlobalContext()] { return global_context->getZooKeeper(); };
            return std::make_shared<BackupCoordinationRemote>(coordination_zk_path, get_zookeeper, !is_internal_backup);
        }
        else
        {
            return std::make_shared<BackupCoordinationLocal>();
        }
    }
    std::shared_ptr<IRestoreCoordination> makeRestoreCoordination(const String & coordination_zk_path, const ContextPtr & context, bool is_internal_backup)
    {
        if (!coordination_zk_path.empty())
        {
            auto get_zookeeper = [global_context = context->getGlobalContext()] { return global_context->getZooKeeper(); };
            return std::make_shared<RestoreCoordinationRemote>(coordination_zk_path, get_zookeeper, !is_internal_backup);
        }
        else
        {
            return std::make_shared<RestoreCoordinationLocal>();
        }
    }
    
    template <typename CoordinationType>
    void sendExceptionToCoordination(std::shared_ptr<CoordinationType> coordination, const String & current_host, const Exception & exception)