Display more accurate progress information

[osteele]

The progress bar currently displays r/(r+1), once r responses have been received. This is at least monotonically increasing.

Ideally, it would display r/n, where n is the total number of responses. But we can't know n in advance.

Alternatively, d/(currentTime-firstRequestTime) could be a better estimate, where d = maxTime(currentRequest), firstRequestTime = maxTime(firstRequest), and maxTime(request) = max(data.clientModified, data.serverModified for data in currentRequest.items).