Slow down polling on API rate limit errors

[scionaltera]

All exchanges have some kind of rate limit on how frequently you can call their APIs. They all start to throw some kind of exception if you exceed that limit, and some will blacklist your IP temporarily (or permanently if you don't stop). Right now these errors can create a lot of noise in the logs and we don't do anything to stop it from happening.

It would be smart if Arbitrader detected these rate limiting errors and slowed down polling for that exchange to try to avoid triggering them continuously. Best would be if it slowed down a little bit at first, but retained some sort of counter so it could manage the delay and find the fastest rate to poll without triggering the errors.