> (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->keeper_dispatcher_mutex);
    return shared->keeper_dispatcher;
}
#endif
void Context::shutdownKeeperDispatcher() const
{
#if USE_NURAFT
    std::lock_guard lock(shared->keeper_dispatcher_mutex);
    if (shared->keeper_dispatcher)
    {
        shared->keeper_dispatcher->shutdown();
        shared->keeper_dispatcher.reset();
    }
#endif
}
void Context::updateKeeperConfiguration([[maybe_unused]] const Poco::Util::AbstractConfiguration & config)
{
#if USE_NURAFT
    std::lock_guard lock(shared->keeper_dispatcher_mutex);
    if (!shared->keeper_dispatcher)
        return;
    shared->keeper_dispatcher->updateConfiguration(config, getMacros());
#endif
}
zkutil::ZooKeeperPtr Context::getAuxiliaryZooKeeper(const String & name) const