Make all inventory collection operations concurrent

[andrewjstone]

We have a lot more inventory collected since #4753 and all of it is collected serially. We should make it all concurrent.

[hawkw]

If nobody else is planning on working on this, I'd be happy to take a crack at it, I could use a little break from RFD 520. :)

[jgallagher]

Porting over @andrewjstone's comment on a PR:

There is actually a comment regarding keeping things serial:

omicron/nexus/inventory/src/collector.rs

Lines 58 to 63 in 809c454

	// We're about to do a bunch of asynchronous operations. With a
	// combination of async, futures, and some cleverness, we could do much
	// of this in parallel. But this code path is not remotely
	// latency-sensitive. And there's real risk of overloading our
	// downstream services. So we just do one step at a time. This also
	// keeps the code simpler.

I think instead we should probably do it in parallel and limit the concurrency with buffer_unordered or something.

This comment seems like it still applies even though we're collecting more stuff, I think?