Make the hardcoded 1 segment per second download rate a conf option

Make the hardcoded 1 segment per second download rate a conf option. One idea is to have two different rates, one for the usual GET and one for COPY. It might be desirable to encourage the end user to COPY a highly segmented object into a whole object for better performance.

Current hardcoded limit is 1 segment per second to limit an end user turning 1 request into many more very quickly, flooding the cluster. 1 segment per second allows 100M segmented objects to be downloaded at 1Gbit rates. If it was changed to say, 100 segments per second, that would allow 1M segmented objects to be downloaded at 1Gbit rates. But a single user request per second would turn into 100 backend requests per second, so caution!