> (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->buffer_flush_schedule_pool)
    {
        size_t background_buffer_flush_schedule_pool_size = 16;
        if (getConfigRef().has("background_buffer_flush_schedule_pool_size"))
            background_buffer_flush_schedule_pool_size = getConfigRef().getUInt64("background_buffer_flush_schedule_pool_size");
        else if (getConfigRef().has("profiles.default.background_buffer_flush_schedule_pool_size"))
            background_buffer_flush_schedule_pool_size = getConfigRef().getUInt64("profiles.default.background_buffer_flush_schedule_pool_size");
        shared->buffer_flush_schedule_pool = std::make_unique<BackgroundSchedulePool>(
            background_buffer_flush_schedule_pool_size,
            CurrentMetrics::BackgroundBufferFlushSchedulePoolTask,
            "BgBufSchPool");
    }
    return *shared->buffer_flush_schedule_pool;
}
BackgroundTaskSchedulingSettings Context::getBackgroundProcessingTaskSchedulingSettings() const
{
    BackgroundTaskSchedulingSettings task_settings;
    const auto & config = getConfigRef();
    task_settings.thread_sleep_seconds = config.getDouble("background_processing_pool_thread_sleep_seconds", 10);
    task_settings.thread_sleep_seconds_random_part = config.getDouble("background_processing_pool_thread_sleep_seconds_random_part", 1.0);
    task_settings.thread_sleep_seconds_if_nothing_to_do = config.getDouble("background_processing_pool_thread_sleep_seconds_if_nothing_to_do", 0.1);
    task_settings.task_sleep_seconds_when_no_work_min = config.getDouble("background_processing_pool_task_sleep_seconds_when_no_work_min", 10);
    task_settings.task_sleep_seconds_when_no_work_max = config.getDouble("background_processing_pool_task_sleep_seconds_when_no_work_max", 600);
    task_settings.task_sleep_seconds_when_no_work_multiplier = config.getDouble("background_processing_pool_task_sleep_seconds_when_no_work_multiplier", 1.1);
    task_settings.task_sleep_seconds_when_no_work_random_part = config.getDouble("background_processing_pool_task_sleep_seconds_when_no_work_random_part", 1.0);
    return task_settings;
}
BackgroundTaskSchedulingSettings Context::getBackgroundMoveTaskSchedulingSettings() const