In situations where there is not a valid source or target phantom
node (e.g. when snapping to an edge with a zero weight), a
heap assertion will fail in the MLD alternative search code.
We fix this by checking for empty heaps before proceeding with
the search.
For the MLD algorithm we can partition the NodeID range into boundary
and non-boundary nodes. Since there are only we boundary nodes we can
use the ArrayStorage for those yielding much faster query times.
Rename module partition to partitioner.
This cultivates naming used in existing modules like extractor,
customizer, etc. - noun vs verb (word partition is both though).
The viaroute plugin always expects a route to be there potentially
with invalid edge weight to represent no-route-found. By switching
to the many-route-result for the mld alternatives algorithm we might
return an empty many-route-result invalidating the post-condition.
