Replace hint with OSM segment id

[TheMarex]

Currently we expose a base 64 encoded version of the PhantomNode class in our API as hint property and accept them as hints array on each query. This is useful when connecting multiple services and ensuring consistent snapping. For example a user can first use the match service to obtain hints from a snapped route and then feed them into the trip service to optimize the route. Using coordinates here causes problems since snapping is ambiguous. Consider for example the coordinate for an intersection, it is not clear which of the connection segments should be used.

This introduces a number of problems since the engine::Hint object is part of our C++ API and depends on the PhantomNode which is an internal implementation detail. Furthermore the size of each hint is quite large increasing the response size significantly. To retain the possibility to connecting services we need a stable approach, possibly across datasets.

The most stable thing we have is OSM ids. A pair of OSM node ids identifies a segment uniquely. To capture the fact that a coordinate can be snapped to the middle of the segment, we would also need some sort of progression along the segment.

Proposal

We encode hint values as:

{start}:{target}:{progression}

Example

26753078:2712265859:0.5 would correspond to this coordinate.

Problems to solve

1. We need a way to go from OSM ids to internal node ids
2. We need a way to go from internal node id pairs to edge-based-node ids and segment offset long the compressed geometry

If we could save 2. in RAM it would enable us to also cut a lot of data from the StaticRTree.

[emiltin]

You can have multiple OSM ways between the same two nodes, e.g a way and a square sharing nodes as is the case here: http://www.openstreetmap.org/way/25822563. Would that be a problem?

Related to #610.