MALFORMED PACKET

Problem: The procedure that creates a temporary table (create_tmp_table())
expects a flat list of expressions that can be mapped to columns in the
temporary table. But in case of row constructor expressions, the list is not
flat.

Solution: Remove row expressions in the contextualization phase of equalities
and non-equalities. The factory function for equality Items gets its public
interface broadened to include conjunctions/disjunctions.

The problem also manifests itself for the operators >=, <=, < and >. In this
case the problem lies in the functions split_sum_func() and
split_sum_func2(). These functions are supposed to recursively replace set
functions with Item_aggregate_ref objects (no splitting at all to be seen.)
The functions should obviously keep recursing over rows, so this has been
added (in item.cc).

(cherry picked from commit 45261d5bd12a90b9c409e0efb398d199ad1541b9)