> (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 (name == "profile")
    {
        setCurrentProfile(value.safeGet<String>());
        return;
    }
    settings.set(name, value);
    if (name == "readonly" || name == "allow_ddl" || name == "allow_introspection_functions")
        calculateAccessRights();
}
void Context::applySettingChange(const SettingChange & change)
{
    try
    {
        setSetting(change.name, change.value);
    }
    catch (Exception & e)
    {
        e.addMessage(fmt::format("in attempt to set the value of setting '{}' to {}",
                                 change.name, applyVisitor(FieldVisitorToString(), change.value)));
        throw;
    }
}
void Context::applySettingsChanges(const SettingsChanges & changes)