apiserver: Replace ForModel with pool in ModelStatus

[babbageclunk]

Description of change

Having status use an existing state instead of starting a fresh one (with the attendant workers) is more efficient.

QA steps

Bootstrap and create a few models. Verify that status works.

Bug reference

Part of https://bugs.launchpad.net/juju/+bug/1698702

[babbageclunk]

$$merge$$

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju

[babbageclunk]

Woot

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