> (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.

{
    getSettingsConstraintsAndCurrentProfiles()->constraints.check(settings, changes);
}
void Context::clampToSettingsConstraints(SettingsChanges & changes) const
{
    getSettingsConstraintsAndCurrentProfiles()->constraints.clamp(settings, changes);
}
void Context::checkMergeTreeSettingsConstraints(const MergeTreeSettings & merge_tree_settings, const SettingsChanges & changes) const
{
    getSettingsConstraintsAndCurrentProfiles()->constraints.check(merge_tree_settings, changes);
}
void Context::resetSettingsToDefaultValue(const std::vector<String> & names)
{
    auto lock = getLock();
    for (const String & name: names)
    {
        settings.setDefaultValue(name);
    }
}
std::shared_ptr<const SettingsConstraintsAndProfileIDs> Context::getSettingsConstraintsAndCurrentProfiles() const
{
    auto lock = getLock();
    if (settings_constraints_and_current_profiles)
        return settings_constraints_and_current_profiles;
    static auto no_constraints_or_profiles = std::make_shared<SettingsConstraintsAndProfileIDs>(getAccessControl());
    return no_constraints_or_profiles;
}
String Context::getCurrentDatabase() const