> (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 (external_tables_initializer_callback)
    {
        external_tables_initializer_callback(shared_from_this());
        
        external_tables_initializer_callback = {};
    }
}
void Context::setInputInitializer(InputInitializer && initializer)
{
    if (input_initializer_callback)
        throw Exception("Input initializer is already set", ErrorCodes::LOGICAL_ERROR);
    input_initializer_callback = std::move(initializer);
}
void Context::initializeInput(const StoragePtr & input_storage)
{
    if (!input_initializer_callback)
        throw Exception("Input initializer is not set", ErrorCodes::LOGICAL_ERROR);
    input_initializer_callback(shared_from_this(), input_storage);
    
    input_initializer_callback = {};
}
void Context::setInputBlocksReaderCallback(InputBlocksReader && reader)
{
    if (input_blocks_reader)
        throw Exception("Input blocks reader is already set", ErrorCodes::LOGICAL_ERROR);
    input_blocks_reader = std::move(reader);
}
InputBlocksReader Context::getInputBlocksReaderCallback() const