DVR Playback / play behind live capabilities

[englishm]

Once we have support for Fetch control messages (#35), we'll also want to be able to make use of Fetch'd data to enable features like playing behind the live edge and possibly scrubbing.

Starting in on the design work here is likely to spawn other issues (like perhaps a need for a "timeline track" mapping objects to timestamps on the media timeline). We can create sub-issues and split out discrete portions of the work as the design takes shape.