[server/session] remove json dumps around call_service This backouts changeset b151beea9cb6. With call_service being in-process and synchronous, there's no RPC use case.

[req.find] Use vreg.schema.eschema for eschema lookup If an etype is non-existant, using vreg.schema[etype] raises a confusing error complaining about a non-existing relation. This is because of the implementation of vreg.schema.__getitem__ that look first in entities and then in relations. Using directly vreg.schema.eschema restrict the lookup to etypes only, hence raises a meaningful error when the etype does not exist.