The reported latency values now include the "schedule lag" time, that is,
the time between the transaction's scheduled start time and the time it
actually started. This relates better to a model where requests arrive at a
certain rate, and we are interested in the response time to the end user or
application, rather than the response time of the database itself.
Also, when --rate is used, include the schedule lag time in the log output.
The --rate option is new in 9.4, so backpatch to 9.4. It seems better to
make this change in 9.4, while we're still in the beta period, than ship a
9.4 version that calculates the values differently than 9.5.
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