> (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.

#include <Access/ExternalAuthenticators.h>
#include <Access/GSSAcceptor.h>
#include <IO/ResourceManagerFactory.h>
#include <Backups/BackupsWorker.h>
#include <Dictionaries/Embedded/GeoDictionariesLoader.h>
#include <Interpreters/EmbeddedDictionaries.h>
#include <Interpreters/ExternalDictionariesLoader.h>
#include <Functions/UserDefined/ExternalUserDefinedExecutableFunctionsLoader.h>
#include <Functions/UserDefined/IUserDefinedSQLObjectsLoader.h>
#include <Functions/UserDefined/createUserDefinedSQLObjectsLoader.h>
#include <Interpreters/ExpressionActions.h>
#include <Interpreters/ProcessList.h>
#include <Interpreters/InterserverCredentials.h>
#include <Interpreters/Cluster.h>
#include <Interpreters/InterserverIOHandler.h>
#include <Interpreters/SystemLog.h>
#include <Interpreters/SessionLog.h>
#include <Interpreters/Context.h>
#include <Interpreters/DDLWorker.h>
#include <Interpreters/DDLTask.h>
#include <Interpreters/Session.h>
#include <Interpreters/TraceCollector.h>
#include <IO/ReadBufferFromFile.h>
#include <IO/UncompressedCache.h>
#include <IO/MMappedFileCache.h>
#include <IO/WriteSettings.h>
#include <Parsers/ASTCreateQuery.h>
#include <Parsers/ParserCreateQuery.h>
#include <Parsers/parseQuery.h>
#include <Parsers/ASTAsterisk.h>