[FIX] res.partner: performance of name_get()

Rev. 93e9a4dc tried to speed up
the 9.0 name_get() of res.partner by only calling fields_get()
once for the whole name_get, instead of calling it lazily.

This may have worked for name_get calls on multiple unnamed
addresses (name is NULL), but this is a rare occurrence
in practice - while now every single name_get() call would
incur the full cost of the fields_get() call.

That fields_get() call is O(n) in the number of fields,
and there can be quite a few for res.partner.
Calling it only for the specific 'type' field can easily be
about 40x faster than without it.

And since name_get() is called for each Many2One field
of each record being read (due to convert_to_read()), reading
a batch of records with partner fields had become quite
a bit slower!

Avoiding this cost entirely with lazy fields_get() calls
saves quite a lot of time for the common cases. And now
that fields_get() specifies a single field, it is cheap
enough to be called within the loop.

Non-scientific benchmark: account.invoice.search_read() in
the list view of invoices took 500ms for 25 invoices
_before_ this fix, and 90ms after!