pgbench progress with timestamp

It is not easy to compare events on a pgbench runs (oops, the tps is down) 
with for instance events in postgres log, so as to figure out what may 
have cause a given glitch.

This patches adds an option to replace the "time since pgbench run 
started" with a timestamp in the progress report so that it is easier to 
compare timelines.

Use milliseconds for consistency with the '%n' log_prefix patch currently 
submitted by Tomas Vondra in the CF.
  sh> ./pgbench -P 1 -N -T 100 -c 2  starting vacuum...end.  progress: 1.0 s, 546.0 tps, lat 3.619 ms stddev 4.426
progress:2.0 s, 575.0 tps, lat 3.480 ms stddev 1.705
 
  sh> ./pgbench -P 1 --progress-timestamp -N -T 100 -c 2  starting vacuum...end.  progress: 1440328800.064 s, 549.0
tps,lat 3.602 ms stddev 1.698  progress: 1440328801.064 s, 570.0 tps, lat 3.501 ms stddev 1.704  ...
 

-- 
Fabien.