[FIXED JENKINS-21618] Ensure that the LoadStatistics return a self-consistent result.

The original method of computing load statistics would compute the total and idle counts independently
which could lead to counting errors while jobs started in between the different state counting operations.
This change switches to returning a `LoadStatisticsSnapshot` so that callers will get a single consistent
view of the counts which was valid for at least one point in time during the collection of the snapshot.