rgw/kafka: do not destroy the connection on errors

as well as other simplifications:
* do not store temporary configuration in the connection object. just
  use as a local variable
* do not create a connection without a producer

other improvements:
* copy to a local list before publishing
* convert internal error codes to errno

Fixes: https://tracker.ceph.com/issues/66017

Signed-off-by: Yuval Lifshitz <[email protected]>

Author: yuvalif