box: apply space upgrade function before updating tuple

If the space is being upgraded, UPDATE/UPSERT requests matching the new
space format should work. However, currently they wouldn't work unless
the target tuple has already been upgraded, because we don't apply the
upgrade function to a tuple fetched from the space before applying
UPDATE operations. Fix this.

Follow-up commit 21e2def9 ("box: introduce result processor").
Needed for https://github.com/tarantool/tarantool-ee/issues/111

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