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. - 

const ASTPtr & partition,
    const StorageMetadataPtr & metadata_snapshot,
    const String & from_,
    bool fetch_part,
    ContextPtr query_context)
{
    Macros::MacroExpansionInfo info;
    info.expand_special_macros_only = false; 
    info.table_id = getStorageID();
    info.table_id.uuid = UUIDHelpers::Nil;
    auto expand_from = query_context->getMacros()->expand(from_, info);
    String auxiliary_zookeeper_name = extractZooKeeperName(expand_from);