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_ast,
    const StorageMetadataPtr & metadata_snapshot,
    const String & with_name,
    ContextPtr local_context,
    TableLockHolder &)
{
    return freezePartitionsByMatcher(getPartitionMatcher(partition_ast, local_context), metadata_snapshot, with_name, local_context);
}
PartitionCommandsResultInfo MergeTreeData::freezeAll(