vinyl: skip needless checks for duplicates on INSERT

If a unique index includes all parts of another unique index, we can
skip the check for duplicates for it on INSERT. Let's mark all such
indexes with a special flag on CREATE/ALTER and optimize out the check
if the flag is set. If there are two indexes that index the same set
of fields, check uniqueness for the one with a lower id, because it
is likelier to have have a "warmer" cache.

Closes #3154