state: Use StatePool instead of ForModel in AllModelWatcher

[mjs]

This is a forward port of #7512.

Description of change

ForModel is inefficient. Starting a AllModelWatcher on a controller with many models would have had a significant resource impact.

Also a drive-by fix for an incorrect call in state/logdb.

QA steps

Bootstrapped a new controller and then connected using this client: https://gist.github.com/mjs/d9d016433e4348b4aa990a8f81d65d03

Deployed software in multiple models and ensured that all expected changes were observed.

Documentation changes

N.A.

Bug reference

https://bugs.launchpad.net/juju/+bug/1698701

[mjs]

$$merge$$

[jujubot]

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

[jujubot]

Build failed: Tests failed
build url: http://juju-ci.vapour.ws:8080/job/github-merge-juju/11152

[jameinel]

is there a reason you chose to forward port this instead of merging 2.2 into develop? If we keep 2.2 synced with develop, then we don't have to track individually what did (and what we forgot) to bring into both. Also, I wonder if ForModel needs a name change. StartNewStateForModel ? Something that doesn't make it look like exactly what you want but actually has a bunch of bad side effects. John =:->

…

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[mjs]

Abandoning this in favour of a merge