Ratelimit helper does not support multi-threading or async

[judahrand]

It seems to me that the ratelimiting helper function found in singer.utils.ratelimit is rather limited. Perhaps we should replace it with an implementation using this rather excellent package which does support multi threading. There was also a PR open which added support for async but for some reason it was closed (tomasbasham/ratelimit#35). Maybe if lack of maintenance is a concern the whole package should be forked and maintained as part of the Singer.io project? It seems like a universally useful package!