Update retry algorithm to be more robust

[egrace479]

That being said, retry algorithms (at least robust ones) for internet protocols are normally written with an exponential escalation of wait time (such as 1, 2, 4, 8, 16, 32, .. seconds). In that case, a user may want to specify at which point to give up and log a failure, for example --max-retries and/or --max-wait.

Originally posted by @hlapp in #1 (comment)

reply: It's waiting after a failed attempt if the response is any of the following: 429, 500, 502, 503, 504. It doesn't have a wait between successful downloads. It has a max number of times to retry on the designated responses, but otherwise just logs the response in the error log (along with the index, filename, and url).

Setting a maximum wait time on a request would probably be a good idea as well. urllib3.request seems to handle much of this when also passed a Retry object. @thompsonmj had also suggested HTTPAdapter as an option that also uses Retry.

Seems reasonable to use HTTPAdapter, since it's already using requests. Must also consider streaming interruption, as noted here.