mtev_json_object ref counting should be atomic

[rileyberton]

extern struct mtev_json_object* mtev_json_object_get(struct mtev_json_object *jso)
{
  if(jso) {
    jso->_ref_count++;
  }
  return jso;
}
extern void mtev_json_object_put(struct mtev_json_object *jso)
{
  if(jso) {
    jso->_ref_count--;
    if(!jso->_ref_count) jso->_delete(jso);
  }
}

Object shared among many threads, get/put pattern results in a bad ref count and either a leak or a crash when jso is freed while there are still references to it.

Ref counting should probably be atomic (ck_pr_inc/dec_64 or similar)