> (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 (isGlobalContext())
        throw Exception(ErrorCodes::LOGICAL_ERROR, "Global context cannot have scalars");
    return scalars.contains(name);
}
void Context::addQueryAccessInfo(
    const String & quoted_database_name,
    const String & full_quoted_table_name,
    const Names & column_names,
    const String & projection_name,
    const String & view_name)
{
    if (isGlobalContext())
        throw Exception(ErrorCodes::LOGICAL_ERROR, "Global context cannot have query access info");
    std::lock_guard<std::mutex> lock(query_access_info.mutex);
    query_access_info.databases.emplace(quoted_database_name);
    query_access_info.tables.emplace(full_quoted_table_name);
    for (const auto & column_name : column_names)
        query_access_info.columns.emplace(full_quoted_table_name + "." + backQuoteIfNeed(column_name));
    if (!projection_name.empty())
        query_access_info.projections.emplace(full_quoted_table_name + "." + backQuoteIfNeed(projection_name));
    if (!view_name.empty())
        query_access_info.views.emplace(view_name);
}
void Context::addQueryFactoriesInfo(QueryLogFactories factory_type, const String & created_object) const
{
    if (isGlobalContext())
        throw Exception(ErrorCodes::LOGICAL_ERROR, "Global context cannot have query factories info");
    std::lock_guard lock(query_factories_info.mutex);
    switch (factory_type)