chore: simplify `DNSUpstreamController` and `DNSUpstream` resource

[DmitriyMV]

This PR does those things:

• Fixes race condition where controller could potentially modify upstream, while other controller is copying its internals to the slice.
• Simplifies run function in DNSUpstreamController by removing all Idx handling.
• Removes Idx field from DNSUpstream. Upstreams are now sorted by their id with №X prefix.
• Proxy Stop is now called from the finalizer. In combination with iterators, this ensures that we only stop upstream when it's fully unreachable.

[DmitriyMV]

/m