Store walking directions for transfers in long distance mode

[hannesj]

Currently only the geometry is stored in the TransferEdge. If the clasic mode is dropped, we need a way to store full walking directions in those edges.

[demory]

This feature is a priority for TriMet as part of their migration to OTP 1.0; adding to the 1.1 milestone. @abyrd has agreed to work on it.