Dead letter queue input should only use timestamp as start date if no offset already stored.

[robbavey]

The current behavior of the DLQ input is, every time logstash starts, to always begin processing events as soon as possible after the startTimestamp property provided in the configuration field, if the field is set, regardless of whether offsets have been stored after that event.

The default behavior for the plugin should be to honor offsets stored. If the user wishes to begin processing at the 'startTimestamp' value after restart each time, then the user should set the 'commitOffsets' flag to false