> (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 shared->moves_executor;
}
OrdinaryBackgroundExecutorPtr Context::getFetchesExecutor() const
{
    return shared->fetch_executor;
}
OrdinaryBackgroundExecutorPtr Context::getCommonExecutor() const
{
    return shared->common_executor;
}
IAsynchronousReader & Context::getThreadPoolReader(FilesystemReaderType type) const
{
    const auto & config = getConfigRef();
    auto lock = getLock();
    switch (type)
    {
        case FilesystemReaderType::ASYNCHRONOUS_REMOTE_FS_READER:
        {
            if (!shared->asynchronous_remote_fs_reader)
            {
                auto pool_size = config.getUInt(".threadpool_remote_fs_reader_pool_size", 100);
                auto queue_size = config.getUInt(".threadpool_remote_fs_reader_queue_size", 1000000);
                shared->asynchronous_remote_fs_reader = std::make_unique<ThreadPoolRemoteFSReader>(pool_size, queue_size);
            }
            return *shared->asynchronous_remote_fs_reader;
        }
        case FilesystemReaderType::ASYNCHRONOUS_LOCAL_FS_READER:
        {
            if (!shared->asynchronous_local_fs_reader)