db method to get all non-stopped workers

[djmitche]

services/worker-manager/src/worker-scanner.js needs to scan non-stopped workers to see what they're up to. Right now, it scans all workers and just ignores those with a 'stopped' status, but that's pretty slow. Let's write a function that returns just the necessary columns, and add an index on state while we're at it.

[djmitche]

Note that this should not ignore stopping!

[djmitche]

In our discussion we were thinking of using a different DB function for src/provisioner.js, but looking at the seen function there, it

• sums capacity by state and worker pool
• tracks seen providerIds for each worker pool
• only cares about non-stopped workers
  It's probably easier to continue to do this in JS with the updated method described in this issue, than to implement a custom DB function that does all of the above.

[imbstack]

The fact that currently we're looping over the workers table to calculate the existing capacity is probabyl causing us to over-provision slightly when the table is modified by the scanner concurrently to us counting in the provisioner. Let's bump the priority of doing this query as described here.

[djmitche]

@imbstack should this end up in the current sprint?

[imbstack]

I didn't add it at first since it seemed slightly off-topic and doesn't fit nicely into either epic but I think it will happen as part of my refactor so I'll add it.