Explore multi-device timeline sync protocol with sub-10ms precision

[markafoltz]

@chrisn mentioned in #80 (comment)

I think that's fine. The original intent behind the suggestion was for OSP to potentially support tightly synchronized playback between agents (e.g., sub 10ms). That would require an additional timeline sync protocol to be added to OSP, which would need research and investigation, so I would leave as future work if there's interest, not a v1 spec feature.

ETSI TS 103 286-2 is the relevant spec for the HbbTV companion screen synchronzation capability, sections 4.3.4 and 4.3.5.

This issue is to explore adding this as an extension to the core OSP protocol.