heartbeat responses on COPY

[John Dickinson]

Just like bulk requests and SLO manifest writes, COPY requests involve lots of server-side work before any response is sent to the client. We've got the ability to send back heartbeat messages to the client on bulk requests and SLO manifest writes so that the client doesn't time out. We should add this to COPY responses too.

(Note: this was seen with HAProxy in front of Swift. The "fix" was to raise the HAProxy timeout from 30 seconds to something like 10 minutes.)