SMSD retry backoff on full dispatch failure

When all dispatchers fail to dispatch an SMS, smsd will retry at the next queue read (which implies a 30 second delay, using default values). A total dispatch failure is a CRITICAL level error, and the loghandler will notify the NAV admin via e-mail.

A long-lasting error state will result in a lot of e-mail spam for the administrator. This should be mitigated by introducing a backoff mechanism for retries. For each completely failed dispatch attempt, the retry delay should increase by a factor X. On successful dispatch, the delay should be reduced to its original setting.