When I queried `system.metrics` table, I found some negative values which hint to some potential internal overflows.

```
SELECT *
FROM system.metrics
FORMAT Vertical

...

Row 31:
───────
metric:      MemoryTrackingInBackgroundProcessingPool
value:       -580863171
description: Total amount of memory (bytes) allocated in background processing pool (that is dedicated for backround merges, mutations and fetches). Note that this value may include a drift when the memory was allocated in a context of background processing pool and freed in other context or vice-versa. This happens naturally due to caches for tables indexes and doesn't indicate memory leaks.

Row 32:
───────
metric:      MemoryTrackingInBackgroundMoveProcessingPool
value:       0
description: Total amount of memory (bytes) allocated in background processing pool (that is dedicated for backround moves). Note that this value may include a drift when the memory was allocated in a context of background processing pool and freed in other context or vice-versa. This happens naturally due to caches for tables indexes and doesn't indicate memory leaks.

Row 33:
───────
metric:      MemoryTrackingInBackgroundSchedulePool
value:       -4461308831
description: Total amount of memory (bytes) allocated in background schedule pool (that is dedicated for bookkeeping tasks of Replicated tables).

...

56 rows in set. Elapsed: 0.003 sec.
```

Server version: 20.8.2.3.
const String & session_id,
        Context & context,
        std::chrono::steady_clock::duration timeout,
        bool throw_if_not_found)
    {
        std::unique_lock lock(mutex);
        auto & user_name = context.client_info.current_user;
        if (user_name.empty())
            throw Exception("Empty user name.", ErrorCodes::LOGICAL_ERROR);
        Key key(user_name, session_id);
        auto it = sessions.find(key);
        if (it == sessions.end())
        {
            if (throw_if_not_found)
                throw Exception("Session not found.", ErrorCodes::SESSION_NOT_FOUND);
            
            it = sessions.insert(std::make_pair(key, std::make_shared<NamedSession>(key, context, timeout, *this))).first;
        }
        else if (it->second->key.first != context.client_info.current_user)
        {
            throw Exception("Session belongs to a different user", ErrorCodes::LOGICAL_ERROR);
        }
        
        const auto & session = it->second;
        if (!session.unique())
            throw Exception("Session is locked by a concurrent client.", ErrorCodes::SESSION_IS_LOCKED);
        session->context.client_info = context.client_info;
        return session;
    }
    void releaseSession(NamedSession & session)