> (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->message_broker_schedule_pool)
    {
        size_t background_message_broker_schedule_pool_size = 16;
        if (getConfigRef().has("background_message_broker_schedule_pool_size"))
            background_message_broker_schedule_pool_size = getConfigRef().getUInt64("background_message_broker_schedule_pool_size");
        else if (getConfigRef().has("profiles.default.background_message_broker_schedule_pool_size"))
            background_message_broker_schedule_pool_size = getConfigRef().getUInt64("profiles.default.background_message_broker_schedule_pool_size");
        shared->message_broker_schedule_pool = std::make_unique<BackgroundSchedulePool>(
            background_message_broker_schedule_pool_size,
            CurrentMetrics::BackgroundMessageBrokerSchedulePoolTask,
            "BgMBSchPool");
    }
    return *shared->message_broker_schedule_pool;
}
ThrottlerPtr Context::getReplicatedFetchesThrottler() const