> (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 = getLock();
    return current_database;
}
String Context::getInitialQueryId() const
{
    return client_info.initial_query_id;
}
void Context::setCurrentDatabaseNameInGlobalContext(const String & name)
{
    if (!isGlobalContext())
        throw Exception("Cannot set current database for non global context, this method should be used during server initialization",
                        ErrorCodes::LOGICAL_ERROR);
    auto lock = getLock();
    if (!current_database.empty())
        throw Exception("Default database name cannot be changed in global context without server restart",
                        ErrorCodes::LOGICAL_ERROR);
    current_database = name;
}
void Context::setCurrentDatabase(const String & name)
{
    DatabaseCatalog::instance().assertDatabaseExists(name);
    auto lock = getLock();
    current_database = name;
    calculateAccessRights();
}
void Context::setCurrentQueryId(const String & query_id)