Requesting segments vs. full route

See also #68 Allow straight lines (comment)

Differences between assembling a route with multiple requests for each segment between two waypoints and requesting the whole route at once with all waypoints:

• different route in case of "island-supression" because "waypoint matching" is re-used for single request (brouter#264), or error (#465)
• no height when via point is on bridge/tunnel (brouter#70)
• no continuous elevation buffer, cost can differ for exact same route with different via position
• turn instruction when via point is exactly on junction node? (#356, brouter#282)
• (initially, start and end point of adjoining segments were not guaranteed to match when requested individually)