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. - 

{
    String source_metadata_version_str;
    bool metadata_version_exists = zookeeper->tryGet(source_path + "/metadata_version", source_metadata_version_str);
    if (!metadata_version_exists)
    {
        
        
        LOG_WARNING(log, "Node {} does not exist. "
                         "Most likely it's because too old version of ClickHouse is running on replica {}. "
                         "Will not check metadata consistency",
                         source_path + "/metadata_version", source_replica);
        return;
    }
    Int32 source_metadata_version = parse<Int32>(source_metadata_version_str);
    if (metadata_version == source_metadata_version)
        return;