[REF] models: make the API of `RecordCache` more consistent

Make `RecordCache` work on a single record only, and implement a mapping from
field names (only) to values.

Also add explicit methods on `RecordCache` to wrap special behavior in cache,
and simplify all special values as a wrapper for a getter function.

The record cache may contain regular and special values.  Modify `RecordCache`
so that only methods ending with `_value` check for regular values.  The
dictionary methods considers both regular and special values as equivalent.

    name in record._cache               # test if cache has a value
    iter(record._cache)                 # iterate on values
    record._cache[name]                 # get value
    record._cache.get(name)             # get value or default

    record._cache.has_value(name)       # test if cache has a regular value
    record._cache.get_value(name)       # get regular value or default

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