Fix StopWatch's stopInstant match stopTimeNanos when split is called

[larshelge]

This PR improves consistency in StopWatch. In StopWatch two variables are used to represent the stop time, using long/milliseconds and Instant respectively, to prepare for Java 8 removal.

For most operations including start, stop and suspend, the two variables are set together to the approximately same time.

The exception is for the split operation, where only the Instant variable is set. This is inconsistent and is problematic when you extend the StopWatch class with a custom class, as the long/milliseconds variable is private and not accessible from the outside, while the Instant variable is the only accessible variable through the getStopInstant. The getStopTime method delegates to getStopInstant. Calling getStopTime after split is called, without also calling stop, is causing a nullpointer exception.

No AI was used for this PR.

[larshelge]

There is a CI test failure but it appears to be unrelated.

[garydgregory]

Yep, this is a "random" and recurring issue. I'll take a look at any side effects for the PR later today or tomorrow.

[garydgregory]

I merged the PR. TY @larshelge 👍

[larshelge]

Great, really appreciate the quick response @garydgregory!