> (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.

{
    shared->interserver_scheme = scheme;
}
String Context::getInterserverScheme() const
{
    return shared->interserver_scheme;
}
void Context::setRemoteHostFilter(const Poco::Util::AbstractConfiguration & config)
{
    shared->remote_host_filter.setValuesFromConfig(config);
}
const RemoteHostFilter & Context::getRemoteHostFilter() const
{
    return shared->remote_host_filter;
}
UInt16 Context::getTCPPort() const
{
    auto lock = getLock();
    const auto & config = getConfigRef();
    return config.getInt("tcp_port", DBMS_DEFAULT_PORT);
}
std::optional<UInt16> Context::getTCPPortSecure() const
{
    auto lock = getLock();
    const auto & config = getConfigRef();
    if (config.has("tcp_port_secure"))
        return config.getInt("tcp_port_secure");
    return {};
}
void Context::registerServerPort(String port_name, UInt16 port)