> (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 FormatFactory::instance().getInput(name, buf, sample, shared_from_this(), max_block_size, format_settings);
}
OutputFormatPtr Context::getOutputFormat(const String & name, WriteBuffer & buf, const Block & sample) const
{
    return FormatFactory::instance().getOutputFormat(name, buf, sample, shared_from_this());
}
OutputFormatPtr Context::getOutputFormatParallelIfPossible(const String & name, WriteBuffer & buf, const Block & sample) const
{
    return FormatFactory::instance().getOutputFormatParallelIfPossible(name, buf, sample, shared_from_this());
}
double Context::getUptimeSeconds() const
{
    auto lock = getLock();
    return shared->uptime_watch.elapsedSeconds();
}
void Context::setConfigReloadCallback(ConfigReloadCallback && callback)
{
    
    shared->config_reload_callback = std::move(callback);
}
void Context::reloadConfig() const
{
    
    if (!shared->config_reload_callback)
        throw Exception("Can't reload config because config_reload_callback is not set.", ErrorCodes::LOGICAL_ERROR);
    shared->config_reload_callback();
}
void Context::shutdown()