> (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.

{
    return *shared->access_control;
}
const AccessControl & Context::getAccessControl() const
{
    return *shared->access_control;
}
void Context::setExternalAuthenticatorsConfig(const Poco::Util::AbstractConfiguration & config)
{
    auto lock = getLock();
    shared->access_control->setExternalAuthenticatorsConfig(config);
}
std::unique_ptr<GSSAcceptorContext> Context::makeGSSAcceptorContext() const
{
    auto lock = getLock();
    return std::make_unique<GSSAcceptorContext>(shared->access_control->getExternalAuthenticators().getKerberosParams());
}
void Context::setUsersConfig(const ConfigurationPtr & config)
{
    auto lock = getLock();
    shared->users_config = config;
    shared->access_control->setUsersConfig(*shared->users_config);
}
ConfigurationPtr Context::getUsersConfig()
{
    auto lock = getLock();
    return shared->users_config;
}
void Context::setUser(const UUID & user_id_)