[nexus] Consider renaming some DB objects, for clarity

As discussed with @davepacheco offline, there are a couple renames that might improve clarity within the codebase.

Note: These don't necessarily need to correlate with API changes, but given the combination of internal / external state nexus is handling, it might be worthwhile for readability.

• Service -> ServiceInstance: Within the DB schema, "Service" currently refers to a single Zone, executing somewhere. However, under some circumstances, "Serivce" can refer to a collection of "Service Instances", where each is interchangeable, but provides redundancy. For example, each of the CRDB frontends acts like an "instance" for a "service". (RFD 248 has more detail).
  By naming these "ServiceInstances", the ambiguity is lessened.

• Instance -> VmInstance: Similarly, the thing called "Instance" in Nexus currently refers to an "Instance of a VM". Given that the word "instance" is useful in other contexts, disambiguating here may be useful too.

Start with RFD 248 and inspect the Nexus database schema definitions and references for Service and Instance. Determine the complete scope of the proposed ServiceInstance and VmInstance renames, including whether any external or API-facing names are affected. Done means the intended database objects and their references use the clarified names without unintended API changes.