Reduce number of Charmhub requests during deploy

[Ben Hoyt]

As https://pastebin.canonical.com/p/V9sF4X7CpH/ shows, when deploying a single charm, we do 8 requests to Charmhub with async-charm-downloads turned on, or 6 requests with it turned off. Two of these definitely look like duplicates, as we call “install without channel, install with channel, install without channel, install with channel” – not sure if there’s a good reason for that yet.

More details / full log:
- with async: https://pastebin.canonical.com/p/Wc94YppDJ5/
- without async: https://pastebin.canonical.com/p/3Fp53NVzwf/

In addition, we should be able to reduce the number of requests in async mode if we had the “essential metadata” in the first batch. See also Achilleas’s comments on the PRs listed in the "collection of info on serverside juju bundle" Google doc.

[Canonical Juju QA Bot (juju-qa-bot)]

This Medium-priority bug has not been updated in 60 days, so we're marking it Low importance. If you believe this is incorrect, please update the importance.