> (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 USE_NURAFT
    std::lock_guard lock(shared->keeper_dispatcher_mutex);
    if (shared->keeper_dispatcher)
        throw Exception(ErrorCodes::LOGICAL_ERROR, "Trying to initialize Keeper multiple times");
    const auto & config = getConfigRef();
    if (config.has("keeper_server"))
    {
        bool is_standalone_app = getApplicationType() == ApplicationType::KEEPER;
        if (start_async)
        {
            assert(!is_standalone_app);
            LOG_INFO(shared->log, "Connected to ZooKeeper (or Keeper) before internal Keeper start or we don't depend on our Keeper cluster, "
                     "will wait for Keeper asynchronously");
        }
        else
        {
            LOG_INFO(shared->log, "Cannot connect to ZooKeeper (or Keeper) before internal Keeper start, "
                     "will wait for Keeper synchronously");
        }
        shared->keeper_dispatcher = std::make_shared<KeeperDispatcher>();
        shared->keeper_dispatcher->initialize(config, is_standalone_app, start_async, getMacros());
    }
#endif
}
#if USE_NURAFT
std::shared_ptr<KeeperDispatcher> & Context::getKeeperDispatcher() const