[client] Exponential random back-off on 429 and 5xx HTTP responses

Upon receiving a 429 or 5xx HTTP response from the server, as well as any failed attempts to establish a TLS connection should result in any future attempts wait in an exponential back-off before retrying to release pressure from the server.

Acceptance criteria:

• Use truncated exponential backoff strategy on 429 and 5XX HTTP responses and connection errors.
  • mender (client)
  • mender-connect
  • monitor-client