golang connection-refresh leaks clients

[igorbernstein2]

The example tries to close the old client after a lameduckTime:
https://github.com/GoogleCloudPlatform/cloud-bigtable-examples/blob/main/go/connection-refresh/btrefresh/bigtable_rotator.go#L64-L68

However RotatingTable#client is only ever set when RotatingRable is constructed, and the new client is never set:
https://github.com/GoogleCloudPlatform/cloud-bigtable-examples/blob/main/go/connection-refresh/btrefresh/bigtable_rotator.go#L70

So the clean up routine will keep closing the same client, while leaking new ones.
Also when addressing this issue, care must be taken to capture the old client before the gorountine start