> (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();
    user_id = user_id_;
    access = getAccessControl().getContextAccess(
        user_id_,  {},  true, settings, current_database, client_info);
    auto user = access->getUser();
    current_roles = std::make_shared<std::vector<UUID>>(user->granted_roles.findGranted(user->default_roles));
    auto default_profile_info = access->getDefaultProfileInfo();
    settings_constraints_and_current_profiles = default_profile_info->getConstraintsAndProfileIDs();
    applySettingsChanges(default_profile_info->settings);
    if (!user->default_database.empty())
        setCurrentDatabase(user->default_database);
}
std::shared_ptr<const User> Context::getUser() const
{
    return getAccess()->getUser();
}
String Context::getUserName() const
{
    return getAccess()->getUserName();
}
std::optional<UUID> Context::getUserID() const
{
    auto lock = getLock();
    return user_id;
}
void Context::setQuotaKey(String quota_key_)