Cap TTFB in attribution

[tunetheweb]

Fixes #439

[philipwalton]

I think it'd be better to extract the logic in the ofTTFB() into a helper in /src/lib/ and then all of these files could reuse the same logic (and also point to the bugs that explain why the logic is there).

[philipwalton]

...thinking about this abit more (and having my memory jogged after looking at the bugs in the source code comments), I think it might be better to not report attribution data in cases where the TTFB value is not trustworthy (since an incorrect TTFB value would impact the accuracy of other sub-part timing values as well).

We don't report the TTFB metric in these cases, and I think it would make sense to do that here as well.

[tunetheweb]

OK re-did this to load from single lib function and also return without an attribution model when the TTFB is invalid.