Use proper collation fun for reduce grouping

We had been using raw collation for grouping keys in a MR view
regardless of the overall collation setting for the view. This patch
defers the construction of the grouping function to the btree code,
where we have access to the correct sorting function.

1 file changed
tree: 9dacddf9b0932bc25c42a766a8b1f47b115ea3b5
  2. include/
  3. priv/
  4. src/
  5. test/