Forward-port metrics for total transaction time and LIFO queue wait time

[divergentdave]

See #4029 and #4029 (comment). These have been landed on release/0.7 already, and need to be implemented (with some small changes) on main.

[tgeoghegan]

When we do this, we also want to plumb mockable clocks into the datastore and task queue modules so that tests can control elapsed time. That's a little tricky, because we don't want to use the existing Clock trait and MockClock, because those operate in terms of DAP times. So we'd need to rig up a new one that returns something akin to std::time::Instant.

[jcjones]

Not done yet, I have the InstantClock trait still to land in a separate PR.