Reported profiling duration is not accurate

[inconshreveable]

The reported profiling duration is actually (time of last query - time of first query), which is not accurate if there is a long period of time between when profiling begins and when the first query is captured (same goes for last query and profile end).

It's not a big deal for now, but it should be changed at some point by possibly modifying the report() API to accept those values.