> (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 (!next_task_callback.has_value())
        throw Exception(ErrorCodes::LOGICAL_ERROR, "Next task callback is not set for query {}", getInitialQueryId());
    return next_task_callback.value();
}
void Context::setReadTaskCallback(ReadTaskCallback && callback)
{
    next_task_callback = callback;
}
MergeTreeReadTaskCallback Context::getMergeTreeReadTaskCallback() const
{
    if (!merge_tree_read_task_callback.has_value())
        throw Exception(ErrorCodes::LOGICAL_ERROR, "Next task callback for is not set for query {}", getInitialQueryId());
    return merge_tree_read_task_callback.value();
}
void Context::setMergeTreeReadTaskCallback(MergeTreeReadTaskCallback && callback)
{
    merge_tree_read_task_callback = callback;
}
PartUUIDsPtr Context::getIgnoredPartUUIDs() const
{
    auto lock = getLock();
    if (!ignored_part_uuids)
        const_cast<PartUUIDsPtr &>(ignored_part_uuids) = std::make_shared<PartUUIDs>();
    return ignored_part_uuids;
}
AsynchronousInsertQueue * Context::getAsynchronousInsertQueue() const