Computing the `cache_key` turned out to be a big factor during the
lifespan of a `BaseModel.mapped` call and a lot of this time is spent
computing the same `cache_key` over and over.

These unnecessary computations can be easily reduced to a couple by
moving the `cache_key` method on the environment (instead of the field)
and by implementing a memo for that method.  The rationale is that the
`cache_key` of a field does not change for a given environment.

The result of this patch is up to 50% faster `Field.__get__` which in
turn means a GLOBAL gain in performance, especially for methods /
functions that rely heavily on `__get__` such as `BaseModel.mapped`.

closes odoo/odoo#42674

Signed-off-by: Adrian Torres (adt) <adt@odoo.com>