> (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 const_cast<Context *>(this)->getExternalUserDefinedExecutableFunctionsLoader();
}
ExternalUserDefinedExecutableFunctionsLoader & Context::getExternalUserDefinedExecutableFunctionsLoader()
{
    std::lock_guard lock(shared->external_user_defined_executable_functions_mutex);
    return getExternalUserDefinedExecutableFunctionsLoaderUnlocked();
}
ExternalUserDefinedExecutableFunctionsLoader & Context::getExternalUserDefinedExecutableFunctionsLoaderUnlocked()
{
    if (!shared->external_user_defined_executable_functions_loader)
        shared->external_user_defined_executable_functions_loader =
            std::make_unique<ExternalUserDefinedExecutableFunctionsLoader>(getGlobalContext());
    return *shared->external_user_defined_executable_functions_loader;
}
EmbeddedDictionaries & Context::getEmbeddedDictionariesImpl(const bool throw_on_error) const
{
    std::lock_guard lock(shared->embedded_dictionaries_mutex);
    if (!shared->embedded_dictionaries)
    {
        auto geo_dictionaries_loader = std::make_unique<GeoDictionariesLoader>();
        shared->embedded_dictionaries = std::make_unique<EmbeddedDictionaries>(
            std::move(geo_dictionaries_loader),
            getGlobalContext(),
            throw_on_error);
    }
    return *shared->embedded_dictionaries;
}
void Context::tryCreateEmbeddedDictionaries(const Poco::Util::AbstractConfiguration & config) const