KNN for nearby command

[tidwall]

It's been often requested to allow for a NEARBY request to order the results by the distance to the target point and to limit by a count rather than a radius.

Including the LIMIT and DISTANCES keyword, and excluding the radius (meters) parameter, Tile38 will guarantee:

• The results will be order by the distance nearest to the target point.
• Each resulting object will include the distance to the target point in meters.
• The number of objects returned will not exceed the value of LIMIT

For example:

NEARBY fleet LIMIT 10 DISTANCES POINT 33.374 -115.829

This will return up to, but not exceeding, 10 objects which are ordered by their distance to the target point 33.374 -115.829.

It may be worth noting that the CURSOR keyword is not compatible with this feature, at least not for the initial release.

[m1ome]

@tidwall i think we can implement right now in a hacky-way, just adding mix of DISTANCE and LIMIT command, and then measure speed. After merge or cancel we can improve it with KNN algo. Sound good?

[tidwall]

@m1ome Yes, that sounds like a good start.

[m1ome]

@tidwall submitted PR #138