> (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();
    for (const SettingChange & change : changes)
        applySettingChange(change);
    applySettingsQuirks(settings);
}
void Context::checkSettingsConstraints(const SettingsProfileElements & profile_elements) const
{
    getSettingsConstraintsAndCurrentProfiles()->constraints.check(settings, profile_elements);
}
void Context::checkSettingsConstraints(const SettingChange & change) const
{
    getSettingsConstraintsAndCurrentProfiles()->constraints.check(settings, change);
}
void Context::checkSettingsConstraints(const SettingsChanges & changes) const
{
    getSettingsConstraintsAndCurrentProfiles()->constraints.check(settings, changes);
}
void Context::checkSettingsConstraints(SettingsChanges & changes) const