Change DLQ classes to be unaware of time

[andsel]

Referring to issue #15562 where a test related to timing constraint proved to be fragile, drive to this request.
The request is to rework the DeadLetterQueueWriter to abstract from physical time, so that test can be done in full control of time.
In such case, the tests doesn't need anymore to create assertions like "this condition has to be met in 1 second but no more that 10", because in that case the test would be synchronous, and not depend on physical time events. Such time events, that trigger the execution of a code block, should be trigger by something external, and in tests it could be used a fake that trigger such events on command.