Currently /trip supports finding round-trip routes where only the
start or end location is fixed. This PR extends this feature to
non-round-trip requests.
We do this by a new table manipulation that simulates non-round-trip
fixed endpoint requests as a round-trip request.
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.
