vinyl: fix primary index uniqueness check being skipped on insert

Since we check uniqueness constraint before inserting anything into the
transaction write set, we have to deal with the situation when a
secondary index doesn't get updated. For example suppose there's tuple
{1, 1, 1} stored in a space with the primary index over the first field
and a unique secondary index over the second field. Then when processing
REPLACE {1, 1, 2}, we will find {1, 1, 1} in the secondary index, but
that doesn't mean that there's a duplicate key error - since the primary
key parts of the old and new tuples coincide, the secondary index
doesn't in fact get updated hence there's no conflict.

However, if the operation was INSERT {1, 1, 2}, then there would be a
conflict - by the primary index. Normally, we would detect such a
conflict when checking the uniqueness constraint of the primary index,
i.e. in vy_check_is_unique_primary(), but there's a case when this
doesn't happen. The point is we can optimize out the primary index
uniqueness constraint check in case the primary index key parts contain
all parts of a unique secondary index, see #3154. In such a case we must
fail vy_check_is_unique_secondary() even if the conflicting tuple has
the same primary key parts.

Fixes commit fc3834c0 ("vinyl: check key uniqueness before modifying
tx write set")

Closes #3643