vinyl: refactor unique check

For the sake of further patches, let's do some refactoring:
 - Rename vy_check_is_unique to vy_check_is_unique_primary and use it
   only for checking the unique constraint of primary indexes. Also,
   make it return immediately if the primary index doesn't need
   uniqueness check, like vy_check_is_unique_secondary does.
 - Open-code uniqueness check in vy_check_is_unique_secondary instead of
   using vy_check_is_unique.
 - Reduce indentation level of vy_check_is_unique_secondary by inverting
   the if statement.