> (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.

{
    return merge_tree_transaction;
}
bool Context::isServerCompletelyStarted() const
{
    auto lock = getLock();
    assert(getApplicationType() == ApplicationType::SERVER);
    return shared->is_server_completely_started;
}
void Context::setServerCompletelyStarted()
{
    auto lock = getLock();
    assert(global_context.lock().get() == this);
    assert(!shared->is_server_completely_started);
    assert(getApplicationType() == ApplicationType::SERVER);
    shared->is_server_completely_started = true;
}
PartUUIDsPtr Context::getPartUUIDs() const
{
    auto lock = getLock();
    if (!part_uuids)
        
        
        
        const_cast<PartUUIDsPtr &>(part_uuids) = std::make_shared<PartUUIDs>();
    return part_uuids;
}
ReadTaskCallback Context::getReadTaskCallback() const