> (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 (config.has("background_move_pool_size"))
        background_move_pool_size = config.getUInt64("background_move_pool_size");
    else if (config.has("profiles.default.background_move_pool_size"))
        background_move_pool_size = config.getUInt64("profiles.default.background_move_pool_size");
    size_t background_fetches_pool_size = 8;
    if (config.has("background_fetches_pool_size"))
        background_fetches_pool_size = config.getUInt64("background_fetches_pool_size");
    else if (config.has("profiles.default.background_fetches_pool_size"))
        background_fetches_pool_size = config.getUInt64("profiles.default.background_fetches_pool_size");
    size_t background_common_pool_size = 8;
    if (config.has("background_common_pool_size"))
        background_common_pool_size = config.getUInt64("background_common_pool_size");
    else if (config.has("profiles.default.background_common_pool_size"))
        background_common_pool_size = config.getUInt64("profiles.default.background_common_pool_size");
    
    shared->merge_mutate_executor = std::make_shared<MergeMutateBackgroundExecutor>
    (
        "MergeMutate",
        background_pool_size,
        background_pool_size * background_merges_mutations_concurrency_ratio,
        CurrentMetrics::BackgroundMergesAndMutationsPoolTask
    );
    LOG_INFO(shared->log, "Initialized background executor for merges and mutations with num_threads={}, num_tasks={}",
        background_pool_size, background_pool_size * background_merges_mutations_concurrency_ratio);
    shared->moves_executor = std::make_shared<OrdinaryBackgroundExecutor>
    (
        "Move",
        background_move_pool_size,
        background_move_pool_size,
        CurrentMetrics::BackgroundMovePoolTask