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)
There are 2 reasonable options here, I think. Store it as minutes as we're basically always going to set it as a while number of minutes. And then convert minutes -> milliseconds in the code where it is used.
OR
Store it as milliseconds and don't have any conversion in the 'code' (only in the config). This is a better option if you use the value in many places. If you only use the value in one place then it makes less difference.

I'd be happy with either of those, really.