want basic internal monitoring of upgrades

[davepacheco]

We discussed this in today's update call. The goal is: assuming we're doing regular internal full runs of a software update, once we set the target release, we want to be able to see what's going on, what's been done, what's left, if it's stuck, etc.

A first cut at this would be having:

• a struct summarizing the version information in inventory: for each kind of component, for each version, how many instances are running that version
• a way to construct this from an inventory collection (or maybe planning input, if we need more information)
• an internal API that exposes this for the latest inventory
• an omdb command to print it out, and optionally poll it (it'd be handy if it showed a "last changed" time)

[davepacheco]

Implemented in #8320.