[FIX] ir_model: invalidate cache when adding new fields

When a new ir.model.field is created, the fields_by_model on the pooler should be reseted to None. This is required for the __init__ method to pool the new list of fields.
Otherwise, the _columns would not contain the new fields and we could not access it without restarting the server (e.g. the installation of a module adds ir.model.fields and use it in the a view.