Align interaction secondary server and primary server in OOB, BC-Proxy and SCD drafts.

[gaperik]

The BC Proxy draft mentions a few alternative to prime the PC Proxy (Head request, prefetch signal from primary to the BC proxy. The other drafts are more vague.

We should align the interaction between the secondary server and primary server for the different kinds of secondary servers.

If we are to select one procedure to use for priming the secondary from the primary, I am leaning towards the LINK header with a resource map telling secondary to pre-fetch.