Fix config duplication detection and throttle goroutine lifecycle.

[timoreimann]

This change fixes two problems:

• Configurations were compared against the persisted configuration, not the latest received; the two may deviate when throttling is active. We introduce a local configuration cache and compare against that.
• The variable managing throttling goroutines was only scoped to the local method, effectively disabling goroutine cleanup and producing a leak. We properly hook up the goroutines to live as long as the server does.

We also refactor the throttling logic into a dedicated struct. While not completely separating the concerns of the server from the throttler (we still need to reference a number of server methods/values inside the throttler), it's a step forward.

Additional changes:

• Pass the stop signal from listenProviders through to the throttler eventually, avoiding the need to close the server for a proper shutdown.
• Exit throttler if ring buffer is closed. (Could happen before stop signal is processed.)
• Test: Return published configs through channel to rule out race conditions.
• Test: Invoke stop channel function earlier to trigger goroutine shutdown.

[timoreimann]

Closing this one in favor of smaller, more digestable PRs. The first one is already out, the others will follow (they are not that critical).