> (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.

auto lock = std::lock_guard(mutex);
            /** Compiled expressions stored in cache need to be destroyed before destruction of static objects.
              * Because CHJIT instance can be static object.
              */
#if USE_EMBEDDED_COMPILER
            if (auto * cache = CompiledExpressionCacheFactory::instance().tryGetCache())
                cache->reset();
#endif
            
            
            
            
            
            
            
            
            
            
            
            dictionaries_xmls.reset();
            user_defined_executable_functions_xmls.reset();
            models_repository_guard.reset();
            delete_system_logs = std::move(system_logs);
            delete_embedded_dictionaries = std::move(embedded_dictionaries);
            delete_external_dictionaries_loader = std::move(external_dictionaries_loader);
            delete_external_user_defined_executable_functions_loader = std::move(external_user_defined_executable_functions_loader);
            delete_user_defined_sql_objects_loader = std::move(user_defined_sql_objects_loader);
            delete_buffer_flush_schedule_pool = std::move(buffer_flush_schedule_pool);
            delete_schedule_pool = std::move(schedule_pool);
            delete_distributed_schedule_pool = std::move(distributed_schedule_pool);