> (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 (!shared->load_marks_threadpool)
    {
        constexpr size_t pool_size = 50;
        constexpr size_t queue_size = 1000000;
        shared->load_marks_threadpool = std::make_unique<ThreadPool>(pool_size, pool_size, queue_size);
    }
    return *shared->load_marks_threadpool;
}
void Context::setIndexUncompressedCache(size_t max_size_in_bytes)
{
    auto lock = getLock();
    if (shared->index_uncompressed_cache)
        throw Exception("Index uncompressed cache has been already created.", ErrorCodes::LOGICAL_ERROR);
    shared->index_uncompressed_cache = std::make_shared<UncompressedCache>(max_size_in_bytes);
}
UncompressedCachePtr Context::getIndexUncompressedCache() const
{
    auto lock = getLock();
    return shared->index_uncompressed_cache;
}
void Context::dropIndexUncompressedCache() const
{
    auto lock = getLock();
    if (shared->index_uncompressed_cache)
        shared->index_uncompressed_cache->reset();
}
void Context::setIndexMarkCache(size_t cache_size_in_bytes)