…rimlet' decision

…er unloading

- Expand the service manager to support initializing the switch zone as
a background task.
- Reserve the Switch Zone address as the second address in the sled's
subnet. The first address is already reserved for the sled agent, but
this ensures that any sled can "become" a scrimlet without needing to
provision a new IP through Nexus.
  - This has some fall-out effects - we bump all the other addresses up,
  which are currently statically allocated.
- Miscellaneous cleanups
  - Identify zones by a "ZoneType" enum, rather than a name.
  - Incorporate the portions of
  #1820 responsible for
  verifying links and devices when launching zones.

Fixes oxidecomputer/minimum-upgradable-product#16
Part of oxidecomputer/minimum-upgradable-product#18