Decide if obstime transformations of coordinates with velocities should result in motion

This issues is a follow-up discussion to an unresolved question from #6219 (in that PR we are punting on the question by raising an error).

The starting point for the topic is https://github.com/astropy/astropy/pull/6219#issuecomment-309399689 - @StuartLittlefair's position is that if you transform to a frame with a different `obstime`, a coordinate with velocities should be moved to a different position based on its velocity vector. @StuartLittlefair added more in https://github.com/astropy/astropy/pull/6219#issuecomment-310907959 and https://github.com/astropy/astropy/pull/6219#issuecomment-310908691 and @mhvk agreed in https://github.com/astropy/astropy/pull/6219#issuecomment-310911108

By contrast, I think this is *not* what we want and will be extremely confusing and inconsistent with the other coordinate machinery - see https://github.com/astropy/astropy/pull/6219#issuecomment-310881737. In https://github.com/astropy/astropy/pull/6219#issuecomment-310906282 and https://github.com/astropy/astropy/pull/6219#issuecomment-310908439 @adrn agreed.

This issue is hence a placeholder, because this is a somewhat significant philosophical question for `coordinates`, so we should resolve it for the next version.

Start by reading the linked discussion in pull request #6219, including the referenced comments, and compare the two positions on obstime transformations in coordinates with velocities. Done means reaching and recording a decision for the next version about whether the transformation should move the coordinate based on its velocity.

Written by the indexing model from the issue text.