> (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->replicated_fetches_throttler)
        shared->replicated_fetches_throttler = std::make_shared<Throttler>(
            settings.max_replicated_fetches_network_bandwidth_for_server);
    return shared->replicated_fetches_throttler;
}
ThrottlerPtr Context::getReplicatedSendsThrottler() const
{
    auto lock = getLock();
    if (!shared->replicated_sends_throttler)
        shared->replicated_sends_throttler = std::make_shared<Throttler>(
            settings.max_replicated_sends_network_bandwidth_for_server);
    return shared->replicated_sends_throttler;
}
ThrottlerPtr Context::getRemoteReadThrottler() const