[REF] fields: change API and semantics of converter methods

The cache data format has been split between a cache format and a record format
(returned by accessing a field on a record).  We have the following converters:

                                   ANY FORMAT
                                       |
                                       | convert_to_cache
                                       v
                                  CACHE FORMAT
                                       |
                                       | convert_to_record
                                       v
                                 RECORD FORMAT
                                       |
                                       | convert_to_read
                                       | convert_to_write
                                       | convert_to_onchange
                                       | convert_to_export
                                       | convert_to_display_name
                                       v
                                 OTHER FORMATS

Modify the API of the converter methods to make them uniform (pass value and
record), and adapt their documentation.

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