> (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.

{
    auto lock = getLock();
    if (!shared->ddl_worker)
    {
        if (!hasZooKeeper())
            throw Exception("There is no Zookeeper configuration in server config", ErrorCodes::NO_ELEMENTS_IN_CONFIG);
        if (!hasDistributedDDL())
            throw Exception("There is no DistributedDDL configuration in server config", ErrorCodes::NO_ELEMENTS_IN_CONFIG);
        throw Exception("DDL background thread is not initialized", ErrorCodes::NO_ELEMENTS_IN_CONFIG);
    }
    return *shared->ddl_worker;
}
zkutil::ZooKeeperPtr Context::getZooKeeper() const
{
    std::lock_guard lock(shared->zookeeper_mutex);
    const auto & config = shared->zookeeper_config ? *shared->zookeeper_config : getConfigRef();
    if (!shared->zookeeper)
        shared->zookeeper = std::make_shared<zkutil::ZooKeeper>(config, "zookeeper", getZooKeeperLog());
    else if (shared->zookeeper->expired())
    {
        Stopwatch watch;
        LOG_DEBUG(shared->log, "Trying to establish a new connection with ZooKeeper");
        shared->zookeeper = shared->zookeeper->startNewSession();
        LOG_DEBUG(shared->log, "Establishing a new connection with ZooKeeper took {} ms", watch.elapsedMilliseconds());
    }
    return shared->zookeeper;
}
namespace
{
bool checkZooKeeperConfigIsLocal(const Poco::Util::AbstractConfiguration & config, const std::string & config_name)