Add the max_attempts limit back in.

The only issue with failing jobs so far is highly active concurrent
writes to the db. This happens when multiple workers process many
RaffleScrapingJobs issued by a NewRafflesJob.

As such, the only problem with this is if all the raffles are:
        a) new to us,
        b) already populated with many participants.

This causes the workers to attempt to insert new Tf2rUsers for all the
participants 'ever'. Avoid this problem by running the initial
scrape:new (and child jobs) with one worker, then increase the number of
workers for subsequently scheduled jobs.

This reverts commit db75e913a8b944d0b4a68e1f0021ee7fcc849140.