[FIX] point_of_sale: performance and invalidation

Function fields, old-api style, that are not stored do not have cache
invalidation mechanisms. This means that they are recomputed every time the
field is accessed.
Due to the way the cache behaves, the records that are present in the cache will
be included when the prefetching is done.

In the scenario of the a POS session being closed, this implies that the cache
will contains more and more account.move.line records and try to prefetch all
(hopefully max 1000) without ever relying on the computed result for the next
access. This makes the computation of fields `amount_total` and `price_subtotal`
very slow, even for orders with a very low number of lines.

Backport the new API migration of these computed field to benefit from depends
mechanism and keep the computed value into cache when needed.

In master these fields should be stored.

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