> (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->application_type = type;
}
void Context::setDefaultProfiles(const Poco::Util::AbstractConfiguration & config)
{
    shared->default_profile_name = config.getString("default_profile", "default");
    getAccessControl().setDefaultProfileName(shared->default_profile_name);
    shared->system_profile_name = config.getString("system_profile", shared->default_profile_name);
    setCurrentProfile(shared->system_profile_name);
    applySettingsQuirks(settings, &Poco::Logger::get("SettingsQuirks"));
    shared->buffer_profile_name = config.getString("buffer_profile", shared->system_profile_name);
    buffer_context = Context::createCopy(shared_from_this());
    buffer_context->setCurrentProfile(shared->buffer_profile_name);
}
String Context::getDefaultProfileName() const
{
    return shared->default_profile_name;
}
String Context::getSystemProfileName() const
{
    return shared->system_profile_name;
}
String Context::getFormatSchemaPath() const
{
    return shared->format_schema_path;
}
void Context::setFormatSchemaPath(const String & path)