Use Queue API for message purging

I noticed that the message module currently manually purges messages on cron, rather than using the Queue API. Using Queue API seems like it would be a more dependable foundation for purging messages, and have the benefit of being time-limited, rather than limited to purging a hard-coded number of messages.

Right now, if you have a site that generates more than 100 messages between cron runs (the hard-coded number to purge), you're out of luck, as the "to purge" list will just grow indefinitely.

However, increasing this number beyond 100 can be risky, since it could cause problems if the purging takes longer than expected at any time (which could cause cron to run into itself).

(It would also be nice to have a batch interface for purging messages retroactively -- imagine wanting to purge hundreds of thousands of messages from the last few years. But I suppose that belongs in its own issue.)