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. - 

* - merge (MERGE),
  * - delete the partition (DROP).
  *
  * Each replica copies (queueUpdatingTask, pullLogsToQueue) entries from the log to its queue (/replicas/replica_name/queue/queue-...)
  *  and then executes them (queueTask).
  * Despite the name of the "queue", execution can be reordered, if necessary (shouldExecuteLogEntry, executeLogEntry).
  * In addition, the records in the queue can be generated independently (not from the log), in the following cases:
  * - when creating a new replica, actions are put on GET from other replicas (createReplica);
  * - if the part is corrupt (removePartAndEnqueueFetch) or absent during the check (at start - checkParts, while running - searchForMissingPart),
  *   actions are put on GET from other replicas;
  *
  * TODO Update the GET part after rewriting the code (search locally).
  *
  * The replica to which INSERT was made in the queue will also have an entry of the GET of this data.
  * Such an entry is considered to be executed as soon as the queue handler sees it.
  *
  * The log entry has a creation time. This time is generated by the clock of server that created entry
  * - the one on which the corresponding INSERT or ALTER query came.
  *
  * For the entries in the queue that the replica made for itself,
  * as the time will take the time of creation the appropriate part on any of the replicas.
  */
class StorageReplicatedMergeTree final : public ext::shared_ptr_helper<StorageReplicatedMergeTree>, public MergeTreeData
{
    friend struct ext::shared_ptr_helper<StorageReplicatedMergeTree>;
public:
    void startup() override;
    void shutdown() override;
    ~StorageReplicatedMergeTree() override;
    std::string getName() const override { return "Replicated" + merging_params.getModeName() + "MergeTree"; }