> (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();
    client_info.quota_key = std::move(quota_key_);
}
void Context::setCurrentRoles(const std::vector<UUID> & current_roles_)
{
    auto lock = getLock();
    if (current_roles ? (*current_roles == current_roles_) : current_roles_.empty())
       return;
    current_roles = std::make_shared<std::vector<UUID>>(current_roles_);
    calculateAccessRights();
}
void Context::setCurrentRolesDefault()
{
    auto user = getUser();
    setCurrentRoles(user->granted_roles.findGranted(user->default_roles));
}
boost::container::flat_set<UUID> Context::getCurrentRoles() const
{
    return getRolesInfo()->current_roles;
}
boost::container::flat_set<UUID> Context::getEnabledRoles() const
{
    return getRolesInfo()->enabled_roles;
}
std::shared_ptr<const EnabledRolesInfo> Context::getRolesInfo() const