deduplication: do not use system_clock as timejumps cause problems

[alexcekay]

This can lock in case of timejumps

time_t timestamp
    = duration_cast<milliseconds>(std::chrono::steady_clock::now() - _start_time).count();
// pop data from front queue, delete corresponding data from multiset
while (!_time_hash_queue.empty()

This can happen due to:

• system_clock is being used, which is not monotonic
• Timestamps are in uint32_t, so there is a cast from the int64_t timestamp
  Both can lead to weird situations where due to the overflow and casting timestamps could be before stored ones.