Update config change endpoint to require no change to the meter controller if the meter ID changes

[dlongley]

[dlongley]

In order to allow the meter controller to change, we will require an additional zcap invocation (two http sigs) or that the invoker not be the controller of the keystore but the meter controller.

[dlongley]

We want to make sure that the meter for a keystore config can't be changed in the current code base -- and we will figure out how to allow it to be changed in the future.

[dlongley]

At present the meter cannot be changed in a keystore config:

https://github.com/digitalbazaar/bedrock-kms-http/blob/v10.0.0/lib/http.js#L195

I'm removing the Priority 1 label for this reason. This issue will be left open for when meter changing becomes possible to do safely in the future.

[dlongley]

Once changing the meter ID is possible, tests should be added. Closing #53 as duplicate. Minimal tests required:

1. Change the meter ID and confirm a new meter is upserted when doing so
2. Negative test that disallows meter with a different controller from the invoker