> (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 cache = tryGetMergeTreeMetadataCache();
    if (!cache)
        throw Exception(
            ErrorCodes::LOGICAL_ERROR,
            "Merge tree metadata cache is not initialized, please add config merge_tree_metadata_cache in config.xml and restart");
    return cache;
}
MergeTreeMetadataCachePtr Context::tryGetMergeTreeMetadataCache() const
{
    return shared->merge_tree_metadata_cache;
}
#endif
void Context::resetZooKeeper() const
{
    std::lock_guard lock(shared->zookeeper_mutex);
    shared->zookeeper.reset();
}
static void reloadZooKeeperIfChangedImpl(const ConfigurationPtr & config, const std::string & config_name, zkutil::ZooKeeperPtr & zk,
                                         std::shared_ptr<ZooKeeperLog> zk_log)
{
    if (!zk || zk->configChanged(*config, config_name))
    {
        if (zk)
            zk->finalize("Config changed");
        zk = std::make_shared<zkutil::ZooKeeper>(*config, config_name, std::move(zk_log));
    }
}
void Context::reloadZooKeeperIfChanged(const ConfigurationPtr & config) const