sql: Duplicate key error for a non-unique index

Adds collation analysis into creating of a composite key for
index tuples.
The keys of secondary index consist of parts defined for index itself
combined with parts defined for primary key.
The duplicate parts are ignored. But the search of duplicates didn't
take the collation into consideration.
If non-unique secondary index contained primary key columns their
parts from the primary key were omitted. This fact caused an issue.

@locker: comments, renames.

Closes #3537