vinyl: do not use column mask as trigger for turning REPLACE into INSERT

If a REPLACE statement was generated by an UPDATE operation that updated
a column indexed by a secondary key, we can turn it into INSERT when the
secondary index is dumped, because there can't be an older statement
with the same key other than DELETE. Currently, we use the statement
column mask to detect such REPLACEs in the write iterator, but I'm
planning to get rid of vy_stmt_column_mask so let's instead introduce
a new statement flag to mark such REPLACEs.