Step Garbage Collection

[Archdoog]

When a user skips a step and is unable to re-route, they may eventually rejoin the route on their own (without instructions or a re-route). In this case, we may actually successfully resume navigation by dropping steps we're certain have been passed (e.g. are well out of range of the user and index behind the current step of intersection.

In concept this will rarely do anything. But there's a chance in certain circumstances it may actually correctly allow them rejoin the route without a reroute.

[michaelkirk]

Solving this might be a more general solution to #285