add monitoring of message size
 - SNetRecGetSize: get size of data_record (size of data fields + tags +
   btags)
 - Currently, support for only c4snet_data_t
    + set function call-backs on data: SNetReferenceSetDataFunc --> set
    function to get size of record fields
    + get size of c4snet_data_t: SNetRefGetDataSize --> call the
    call-back function if set, otherwise, return -1