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 <ext/scope_guard_safe.h>
#include <algorithm>
#include <iomanip>
#include <optional>
#include <set>
#include <thread>
#include <typeinfo>
#include <typeindex>
#include <unordered_set>
#include <filesystem>
namespace fs = std::filesystem;
namespace ProfileEvents
{
    extern const Event RejectedInserts;
    extern const Event DelayedInserts;
    extern const Event DelayedInsertsMilliseconds;
    extern const Event DuplicatedInsertedBlocks;
}
namespace CurrentMetrics
{
    extern const Metric DelayedInserts;
    extern const Metric BackgroundMovePoolTask;
}
namespace
{
    constexpr UInt64 RESERVATION_MIN_ESTIMATION_SIZE = 1u * 1024u * 1024u; 
}
namespace DB
{
namespace ErrorCodes