> (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->zookeeper_mutex);
    shared->zookeeper_config = config;
    reloadZooKeeperIfChangedImpl(config, "zookeeper", shared->zookeeper, getZooKeeperLog());
}
void Context::reloadAuxiliaryZooKeepersConfigIfChanged(const ConfigurationPtr & config)
{
    std::lock_guard lock(shared->auxiliary_zookeepers_mutex);
    shared->auxiliary_zookeepers_config = config;
    for (auto it = shared->auxiliary_zookeepers.begin(); it != shared->auxiliary_zookeepers.end();)
    {
        if (!config->has("auxiliary_zookeepers." + it->first))
            it = shared->auxiliary_zookeepers.erase(it);
        else
        {
            reloadZooKeeperIfChangedImpl(config, "auxiliary_zookeepers." + it->first, it->second, getZooKeeperLog());
            ++it;
        }
    }
}
bool Context::hasZooKeeper() const