[meshing] determine how to deliver the mesh to the author

[rcabanier]

From @thetuvix:

Given that getting to world mesh involves latent processing anyway, it may be less important to eagerly force finished mesh on apps ASAP. Instead, if the mesh metadata object has a .requestMesh() method that returns a promise, that could get us the best of both worlds, allowing apps to request the mesh they care about most earliest.

and

The optimal APIs to allow users to both asynchronously request durable world mesh updates and also synchronously receive per-frame pose-predicted hand mesh updates may differ. We should consider how best to address each of these cases first, and then see if there's common ground worth combining.