Change progress report to show "instantaneous" rate. Also print total time.

Change progress report to show "instantaneous" rate. Also print total time.

Instead of always showing the overall rate, we wait until the end
to show that. Then, on incremental updates we show the rate over the
last increment. This makes it much easier to actually watch what's
happening, (and it's easy to see the efect of xapian's internal
10,000 document flush).