> (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();
    if (this->temp_data_on_disk)
        return this->temp_data_on_disk;
    return shared->temp_data_on_disk;
}
void Context::setTempDataOnDisk(TemporaryDataOnDiskScopePtr temp_data_on_disk_)
{
    auto lock = getLock();
    this->temp_data_on_disk = std::move(temp_data_on_disk_);
}
void Context::setPath(const String & path)
{
    auto lock = getLock();
    shared->path = path;
    if (shared->tmp_path.empty() && !shared->temp_data_on_disk)
        shared->tmp_path = shared->path + "tmp/";
    if (shared->flags_path.empty())
        shared->flags_path = shared->path + "flags/";
    if (shared->user_files_path.empty())
        shared->user_files_path = shared->path + "user_files/";
    if (shared->dictionaries_lib_path.empty())
        shared->dictionaries_lib_path = shared->path + "dictionaries_lib/";
    if (shared->user_scripts_path.empty())
        shared->user_scripts_path = shared->path + "user_scripts/";
}
static void setupTmpPath(Poco::Logger * log, const std::string & path)