Periodically save the message ID to the state.json

[TheTechRobo]

This would be a path to resuming the crawl without using the date filters (which is a bad way to resume since there are the messages loaded when the channel is clicked on included in the dump).

The message ID fetched should preferably be from the top of the loaded messages so it doesn't accidentally skip surrounding messages.

Unlike #6, this would be good for putting inside the task listed as "current" in the state.json so crashed jobs can be quickly and easily resumed.