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. - 

#include <Compression/CompressedReadBuffer.h>
#include <DataStreams/copyData.h>
#include <DataTypes/DataTypeArray.h>
#include <DataTypes/DataTypeDate.h>
#include <DataTypes/DataTypeDateTime.h>
#include <DataTypes/DataTypeEnum.h>
#include <DataTypes/DataTypeLowCardinality.h>
#include <DataTypes/DataTypeNullable.h>
#include <DataTypes/DataTypeUUID.h>
#include <DataTypes/DataTypeTuple.h>
#include <DataTypes/NestedUtils.h>
#include <Formats/FormatFactory.h>
#include <Functions/FunctionFactory.h>
#include <Functions/IFunction.h>
#include <IO/ConcatReadBuffer.h>
#include <IO/Operators.h>
#include <IO/ReadBufferFromMemory.h>
#include <IO/WriteBufferFromString.h>
#include <Interpreters/ExpressionAnalyzer.h>
#include <Interpreters/PartLog.h>
#include <Interpreters/TreeRewriter.h>
#include <Interpreters/inplaceBlockConversions.h>
#include <Interpreters/Context.h>
#include <Interpreters/InterpreterSelectQuery.h>
#include <Parsers/ASTFunction.h>