> (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();
    return settings;
}
void Context::setSettings(const Settings & settings_)
{
    auto lock = getLock();
    auto old_readonly = settings.readonly;
    auto old_allow_ddl = settings.allow_ddl;
    auto old_allow_introspection_functions = settings.allow_introspection_functions;
    settings = settings_;
    if ((settings.readonly != old_readonly) || (settings.allow_ddl != old_allow_ddl) || (settings.allow_introspection_functions != old_allow_introspection_functions))
        calculateAccessRights();
}
void Context::setSetting(std::string_view name, const String & value)
{
    auto lock = getLock();
    if (name == "profile")
    {
        setCurrentProfile(value);
        return;
    }
    settings.set(name, value);
    if (name == "readonly" || name == "allow_ddl" || name == "allow_introspection_functions")
        calculateAccessRights();
}
void Context::setSetting(std::string_view name, const Field & value)