Before this CL, each contact and host was independently
loaded via the ReferenceUnion adapter. Since fields are
processed serially by JAXB, this means worst-case there
were 17 loads, best case 3 (the 3 required contacts) and
usual case 5-6 (some hosts). This CL reduces that to 1
datastore roundtrip in all cases.
A side effect of this CL is the further hollowing-out of
ReferenceUnion, since it no longer is involved in
marshalling at all.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=123712842