BibField: better handling of calculated fields
- Adds new internal key __calculated_functions to store how to update the cached value of the field.
- All the fields marked with @do_not_cache are not longer stored stored inside rec_json, only the key is reserved.
Signed-off-by: Esteban J. G. Gabancho <esteban.gabancho@gmail.com>
Tested-by: Tibor Simko <tibor.simko@cern.ch>