pub trait GenericLMDBCursor<ActualDBData> {
   type Key: ActualDBData::Key+?Sized;
   type Value: ActualDBData::Value;
   // methods..
}
pub trait ActualDBData {
  type Key: ToMdbValue+?Sized;
  type Value: Message+MessageStatic; //protobuffers
  fn get_db_handle<'a>() -> &'a Database;
}
pub struct DocDB;
impl ActualDBData {
  type Key = str;
  type Value = DocProto;
  fn get_db_handle<'a>() -> &'a Database {
        get_cached_doc_handle()
   }
}