This function returns the counted length of this record (the
actual size in bytes of the record on disk when read.) This
value is only valid if the MDBRecord has been loaded from disk.
A helper function for the getFrameBytes function that returns an array
of all the data contained in any extra fields that may be present in
this frame.
Returns the length of this record (in bytes.)
Record lenghts are computed lazily, and are only computed when
needed (a generatePointers by controling MDB file.)