the bug was that in_vector array in Item_func_in was allocated in the
statement arena, not in the table->expr_arena.

revert part of the 5acd391e. Instead, change the arena correctly
in fix_all_session_vcol_exprs().

Remove TABLE_ARENA, that was introduced in 5acd391e to force
item tree changes to be rolled back (because they were allocated in the
wrong arena and didn't persist. now they do)