Since 9fba29ab (memtx: introduce tuple
compare hint) index_get uses key_hint to get a comparison hint for the
key. It requires key type to be validated. However in
space_before_replace index_get was used without key validation on some
execution paths. Now it is fixed and validation is being performed if
needed. Corresponding test case is introduced. Also
space_before_replace() is minor refactored to reduce conditions amount
on a hot path.

Closes #5093