Write a "checkpoint" file to avoid events being replayed

[dpup]

Expected Behavior:

• Running the script as a cron-job shouldn't re-suggest activity based on events that have already been processed.

Actual Behavior:

• Running the script may re-suggest suggestions that have already been dismissed or published in an update.

Suggested Fix:

Write a checkpoint file with the timestamp of the last event that was processed, and only process events newer than that checkpoint.