node-proxy: adjust log levels

node-proxy: adjust log levels

Let's adjust log levels across the project:

- use warning for bad request in the API,
  when thread is not alive and for retry failure,
- use error for OOB load failure,
- use info for backoff interval,
- use debug in send attempts and for member fetch

Signed-off-by: Guillaume Abrioux <gabrioux@ibm.com>
(cherry picked from commit c9b7a306f34de0a44541d736822d2528a5b48966)