Prototype MSC2775: Lazy loading over federation

[callahad]

Large, complex rooms with many servers can take many minutes to join over federation. This is a known issue (#1211).

One possible option for addressing this is described in MSC2775: Lazy loading over federation.

We should prototype it in Synapse to determine its viability.

[richvdh]

I am tracking progress on this work at https://hackmd.io/@richvdh/SkhcKnjKY.