[CBM] [Stats] Error when closing internal stat collectors

What's the issue?

cannot close stat file of ongoing collection

There appears to be an issue in graceful teardown of the 'cbbackupmgr' internal stat collection due to our mixed use of 'context.Context' and 'atomic.LoadUint64'.

1. Stat collection stopped
2. Cancel context
3. Close state gatherers
4. Handle context cancellation and atomically set `done` variable.
5. Check value of `done`

We've only seen this issue occur twice in our Capella environments to date (based on our historical logging) so it seems to be fairly rare.

I suspect this is probably unlikely to happen in the common case - as we've observed - but when there's actually a stat collection running (occurs periodically, >30s) that will block the context cancellation from propagating, as the thread is busy collecting stats.

Steps to Reproduce (Hypothesis)

1. Start backup that is sufficiently large to have stat collection trigger
2. Backup completes while a stat collection is in-progress

What's the fix?
We shouldn't close the workers until the context cancellation has propagated.