PROOF Processing rate fix

This patch is meant to fix an issue causing the PROOF progress bar to get
to 100%, then scroll at 100% for about the same amount of time again before
finishing. The problem was finally tracked down to a failure to update a
TProofProgressStatus object's fLastUpdate field on a worker before it is
sent to the client. The last update time was always the initial time,
causing the reported rate (and progress) to be about a factor of 2 too
large. The patch also includes a minor fix for the scrolling in general,
which is irritating (it can still happen with the bugfix a bit at the end
of the job if the estimate overshoots the total number of entries).

[From: B. Butler, M. Swiatlowski]