> (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.

{
    std::lock_guard lock(shared->storage_policies_mutex);
    return getStoragePolicySelector(lock)->getPoliciesMap();
}
DiskSelectorPtr Context::getDiskSelector(std::lock_guard<std::mutex> & ) const
{
    if (!shared->merge_tree_disk_selector)
    {
        constexpr auto config_name = "storage_configuration.disks";
        const auto & config = getConfigRef();
        auto disk_selector = std::make_shared<DiskSelector>();
        disk_selector->initialize(config, config_name, shared_from_this());
        shared->merge_tree_disk_selector = disk_selector;
    }
    return shared->merge_tree_disk_selector;
}
StoragePolicySelectorPtr Context::getStoragePolicySelector(std::lock_guard<std::mutex> & lock) const
{
    if (!shared->merge_tree_storage_policy_selector)
    {
        constexpr auto config_name = "storage_configuration.policies";
        const auto & config = getConfigRef();
        shared->merge_tree_storage_policy_selector = std::make_shared<StoragePolicySelector>(config, config_name, getDiskSelector(lock));
    }
    return shared->merge_tree_storage_policy_selector;
}
void Context::updateStorageConfiguration(const Poco::Util::AbstractConfiguration & config)