Allow messages to be delivered only once on drain

[brentsnook]

Hi,

Thanks for fake_sns, we've been using it lately for testing a number of services locally. One thing that caught us out was that messages will be repeatedly delivered on subsequent drain requests. The way we are using fake_sns is to run a thread that repeatedly drains and this was resulting in a flurry of messages being delivered to the subscribing queues.

We were thinking of something like a configuration option for only delivering messages once on drain. One approach would be to mark messages in the database once they have been delivered and then filter them in Database#each_deliverable_message.

What do you think?