> (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.

{
    if (!buffer_context) throw Exception("There is no buffer context", ErrorCodes::LOGICAL_ERROR);
    return buffer_context;
}
const EmbeddedDictionaries & Context::getEmbeddedDictionaries() const
{
    return getEmbeddedDictionariesImpl(false);
}
EmbeddedDictionaries & Context::getEmbeddedDictionaries()
{
    return getEmbeddedDictionariesImpl(false);
}
const ExternalDictionariesLoader & Context::getExternalDictionariesLoader() const
{
    return const_cast<Context *>(this)->getExternalDictionariesLoader();
}
ExternalDictionariesLoader & Context::getExternalDictionariesLoader()
{
    std::lock_guard lock(shared->external_dictionaries_mutex);
    return getExternalDictionariesLoaderUnlocked();
}
ExternalDictionariesLoader & Context::getExternalDictionariesLoaderUnlocked()
{
    if (!shared->external_dictionaries_loader)
        shared->external_dictionaries_loader =
            std::make_unique<ExternalDictionariesLoader>(getGlobalContext());
    return *shared->external_dictionaries_loader;
}
const ExternalUserDefinedExecutableFunctionsLoader & Context::getExternalUserDefinedExecutableFunctionsLoader() const