Use medians/percentiles instead of mean travel times

[mattwigway]

Mean travel times have a lot of issues when there are undefined/null values or trips that require excessive waiting (see #162, #163 and opentripplanner/OpenTripPlanner#2148). However, if we use medians, we can cut the search off at any value larger than the largest cutoff we want to use, because the median remains the same (or at least on the same side of our cutoff). We could then rip out the configurable maximum trip length and the reachability threshold, which were statistical hacks anyhow.