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. - 

{
    extern const Metric BackgroundFetchesPoolTask;
}
namespace DB
{
namespace ErrorCodes
{
    extern const int CANNOT_READ_ALL_DATA;
    extern const int NOT_IMPLEMENTED;
    extern const int NO_ZOOKEEPER;
    extern const int INCORRECT_DATA;
    extern const int INCOMPATIBLE_COLUMNS;
    extern const int REPLICA_IS_ALREADY_EXIST;
    extern const int NO_REPLICA_HAS_PART;
    extern const int LOGICAL_ERROR;
    extern const int TOO_MANY_UNEXPECTED_DATA_PARTS;
    extern const int ABORTED;
    extern const int REPLICA_IS_NOT_IN_QUORUM;
    extern const int TABLE_IS_READ_ONLY;
    extern const int NOT_FOUND_NODE;
    extern const int NO_ACTIVE_REPLICAS;
    extern const int NOT_A_LEADER;
    extern const int TABLE_WAS_NOT_DROPPED;
    extern const int PARTITION_ALREADY_EXISTS;
    extern const int TOO_MANY_RETRIES_TO_FETCH_PARTS;
    extern const int RECEIVED_ERROR_FROM_REMOTE_IO_SERVER;
    extern const int PARTITION_DOESNT_EXIST;
    extern const int UNFINISHED;
    extern const int RECEIVED_ERROR_TOO_MANY_REQUESTS;
    extern const int TOO_MANY_FETCHES;