Use estimates for NoRoute in table requests

[danpat]

Issue

When working with very large table requests (1M+ entries), it can be difficult to parse/modify/handle the large response.

A common request we've had from customers is to "never return a null value in a table response" - usually, if no route is found, some estimate can be used instead.

This PR implements a new Table API parameter called fallback_speed. When used, if a null field exists in a table JSON response, it will be replaced by a straight-line estimate between the source/destination. By default, we measure the distance between the input coordinates (supplied in the request), but you can use fallback_coordinate=snapped to use the snapped location instead. Once the distance is found, it is divided by the fallback_speed value to come up with a duration estimate.

Doing this work inside OSRM before constructing the response JSON is significantly more convenient and performant than doing it after the fact dealing with JSON data.

For 1,000,000 cell table requests (which takes about 2-3s on my test machine), this adds approximately 0.7ms if there are no null values to be replaced, and about 30ms if all cells need to be replaced.

Tasklist

• CHANGELOG.md entry (How to write a changelog entry)
• update relevant Wiki pages
• add tests (see testing documentation
• review
• adjust for comments
• cherry pick to release branch