Agg evicting queue

Deprecates #1282 and solves #1214 and #1216.

Behaviour is as follows:
 * Users are free to download
 * If an IP address is "abusing" us with too many aggregator downloads, it gets penalized with a captcha
 * `trustedClients` in configuration can always download without a captcha
 * `maxAggregatedDownloadsInTenMinutes` set to 2 by default (configurable)
When this is ready I think we should have a different configuration in RC so that Kate's IP is not trusted so that she can test the captcha.

We will also need to show this new feature to Kate so she knows about it and help her write a couple of test cases to test that it works.