Fix "exceeding us by" calculation in AggregateStore.Debug statistics.
Fix "exceeding us by" calculation in AggregateStore.Debug statistics.

Prior to this change it:

Was incorrectly based on the current number of slowest traces, rather than our
maximum bound.

Would sometimes print as exceeding us negatively (valid -- but not what the
user would expect).

joeshaw/appdash,bg451/appdash,joeshaw/appdash,joeshaw/appdash,bg451/appdash,bg451/appdash