> (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 getConfigRef().has("zookeeper");
}
bool Context::hasAuxiliaryZooKeeper(const String & name) const
{
    return getConfigRef().has("auxiliary_zookeepers." + name);
}
InterserverCredentialsPtr Context::getInterserverCredentials() const
{
    return shared->interserver_io_credentials.get();
}
void Context::updateInterserverCredentials(const Poco::Util::AbstractConfiguration & config)
{
    auto credentials = InterserverCredentials::make(config, "interserver_http_credentials");
    shared->interserver_io_credentials.set(std::move(credentials));
}
void Context::setInterserverIOAddress(const String & host, UInt16 port)
{
    shared->interserver_io_host = host;
    shared->interserver_io_port = port;
}
std::pair<String, UInt16> Context::getInterserverIOAddress() const
{
    if (shared->interserver_io_host.empty() || shared->interserver_io_port == 0)
        throw Exception("Parameter 'interserver_http(s)_port' required for replication is not specified in configuration file.",
                        ErrorCodes::NO_ELEMENTS_IN_CONFIG);
    return { shared->interserver_io_host, shared->interserver_io_port };
}
void Context::setInterserverScheme(const String & scheme)