Unify services and datasets, external and internal DNS are datasets too

[smklein]

Prelude

This combines:

• [nexus][sled-agent] DNS services are backed by explicit datasets #3022
• [sled-agent] Vectorize the request for dataset zones #3025
• [sled-agent] Unify requests for services and services backed by datasets #3026

Into one PR. Trying to split them out resulted in intermediate states that were not worthwhile debugging - I'd rather invest the energy into fixing this "more-desirable end-state".

What does this PR do

• (Internal and External) DNS services are explicitly backed by datasets, making it possible to "delete and re-initialize" arbitrary zones without data loss. This is necessary for correct behavior in cold boot.
• Merges the concept of "datasets" into "services".
  • Previously, "datasets" were considered services managing data, and "services" were considered services without datasets - the two were totally disjoint. This was a bit arbitrary, and made it more complicated to conform sleds during service management.
  • As one example of an inconsistency, "requesting services" was vectorized ("ask for all services that should be on the sled"), "requesting datasets" was not (it was an ask-for-one-at-a-time API).
  • Now, "services" are universal, and they may optionally manage datasets. This unifies pathways a bit, and simplifies the API.

Fixes #3018
Part of #2978

[smklein]

Rebased around the service bundle changes - this is still ready for review!

[bnaecker]

Looks good to me, a few nits, but nice work overall!