== Interface inconsistency ==
- enum_id member of ftdm_event_t is inconsistent. Most of the time is just for OOB events, the only other
  type of event as of now is FTDM_EVENT_DTMF and is not using the enum_id member. I think we can get rid
  of the FTDM_EVENT_DTMF and create ftdm_dtmf_event_t type instead of reusing ftdm_event_t
  then ftdm_event_t would be renamed to ftdm_oob_event_t and the enum_id renamed to type, then ftdm_span_next_event()
  will only return OOB events

- Deprecate last_error members. 
  It requires a lot of discipline to set the last_error string for every failure.
  It does not add much value to the user either, most of the errors are criptic and
  cannot be shown to end users, we already provide extensive logging for problem
  troubleshooting.