Fix latency calculation when there are \sleep commands in the script.

Fix latency calculation when there are \sleep commands in the script.

We can't use txn_scheduled to hold the sleep-until time for \sleep, because
that interferes with calculation of the latency of the transaction as whole.

Backpatch to 9.4, where this bug was introduced.

Fabien COELHO

Discussion: <alpine.DEB.2.20.1608231622170.7102@lancre>