Topic lookup is executed for every call to Publisher#publish

[brandonc]

https://github.com/kapost/circuitry/blob/master/lib/circuitry/publisher.rb#L64

Seems like we should have some topic ARN cache.

[aemadrid]

This seems like a bottleneck to publishing. Unless I'm missing something it is sending a create topic request for every single publish call. By having an arn cache we can skip creating on each request and then we can catch the exception for a missing topic and recreate it or bubble up.