Symptoms: all threads in BackgroundScheduledPool busy, lot of tasks in replication queue, none of the tasks is executing. 

![Screenshot from 2021-04-16 16-21-44_](https://user-images.githubusercontent.com/1549571/115193796-e767d600-a0ec-11eb-887c-5c9c2412e50c.png)

Related to #16895

Possible workarounds (poor options): increasing the background_schedule_pool_size to huge values, or restart server every time when that happens etc. - 

get_part = [&, address, timeouts, credentials, interserver_scheme]()
        {
            if (interserver_scheme != address.scheme)
                throw Exception("Interserver schemes are different: '" + interserver_scheme
                    + "' != '" + address.scheme + "', can't fetch part from " + address.host,
                    ErrorCodes::INTERSERVER_SCHEME_DOESNT_MATCH);
            return fetcher.fetchPart(
                metadata_snapshot,
                getContext(),
                part_name,
                source_replica_path,
                address.host,
                address.replication_port,
                timeouts,
                credentials->getUser(),
                credentials->getPassword(),
                interserver_scheme,
                to_detached,
                "",
                &tagger_ptr,
                true);
        };
    }
    try
    {
        part = get_part();
        if (!to_detached)
        {
            Transaction transaction(*this);
            renameTempPartAndReplace(part, nullptr, &transaction);