[IMP] models: attach prefetching data to recordsets instead of environments

The prefetching now has a more intuitive and controllable model: every
recordset is linked to a "prefetch object" that tells which record ids to
prefetch for any model.  The prefetch object of a given recordset is shared
with the records obtained by iteration and relational field access.  All other
operations instantiate recordsets with a fresh prefetch object.

    records = model.browse(ids)
    assert records._prefetch is not model._prefetch

    for record in records:
        assert record._prefetch is records._prefetch
        assert record.relation._prefetch is record._prefetch

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