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 String & table_zookeeper_path, const ReplicatedMergeTreeLogEntryData & entry, bool wait_for_non_active)
{
    LOG_DEBUG(log, "Waiting for all replicas to process {}", entry.znode_name);
    auto zookeeper = getZooKeeper();
    Strings replicas = zookeeper->getChildren(fs::path(table_zookeeper_path) / "replicas");
    Strings unwaited;
    for (const String & replica : replicas)
    {
        if (wait_for_non_active || zookeeper->exists(fs::path(table_zookeeper_path) / "replicas" / replica / "is_active"))
        {
            if (!waitForTableReplicaToProcessLogEntry(table_zookeeper_path, replica, entry, wait_for_non_active))
                unwaited.push_back(replica);
        }
        else
        {
            unwaited.push_back(replica);
        }
    }
    LOG_DEBUG(log, "Finished waiting for all replicas to process {}", entry.znode_name);
    return unwaited;
}
Strings StorageReplicatedMergeTree::waitForAllReplicasToProcessLogEntry(
    const ReplicatedMergeTreeLogEntryData & entry, bool wait_for_non_active)
{
    return waitForAllTableReplicasToProcessLogEntry(zookeeper_path, entry, wait_for_non_active);
}
bool StorageReplicatedMergeTree::waitForTableReplicaToProcessLogEntry(