transfers.txt should override BOARD_SLACK_SECONDS and OSM walk distance

[abyrd]

At least for the standard stop-to-stop entries in transfers.txt, and especially for the transfer type 2 ("minimum amount of time between arrival and departure to ensure a connection") we should be overriding the default OSM walk distance calculations. Although the GTFS spec text says "minimum amount of time", I believe the proper interpretation is to completely replace any transfers found by walking through the OSM network: transfers.txt is providing a typical safe amount of time needed to walk between the two stops.

This gets a little tricky because we currently apply a hard lower limit of 60 seconds between alighting and boarding, which would have to be configurable and interact with the transfer entries.

Related to #330. Some of these behaviors may need to be configurable at the routing or network building stage, as not all users will want stations and transfers to be interpreted in the same way. Many of these characteristics must be made optional, but we'll need a mechanism to configure that (#644).

[abyrd]

Strangely BOARD_SLACK_SECONDS appears to only be used in classes for displaying paths, not for routing. This should be described in more detail in the docs.

[abyrd]

Discussed again today - we decided that if transfers.txt is present, especially with transfers of type 2, then overriding any OSM transfers is more correct and should be the default behavior - in fact this probably doesn't even need a switch to disable it unless someone comes up with a concrete need to disable it.