pgbench: Elaborate latency reporting.

Isolate transaction latency (elapsed time between submitting first
command and receiving response to last command) from client-side delays
pertaining to the --rate schedule.  Under --rate, report schedule lag as
defined in the documentation.  Report latency standard deviation
whenever we collect the measurements to do so.  All of these changes
affect --progress messages and the final report.

Fabien COELHO, reviewed by Pavel Stehule.

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