Re-designed a way by that Item_trigger_field objects are arranged in memory.

Item_trigger_field objects created on parsing a trigger's statement
is now stored in a per statement list. All lists of Item_trigger_field
objects created on parsing the whole trigger's body are organized
in the structure "list of lists". So, use binary cycle to iterate every
Item_trigger_field object created on parsing a trigger body.

To organize the data structure 'list of lists' the new data member
  Item_trigger_field::next_trig_field_list
is introduced that links lists in this hierarchy structure.

This re-design is performed in order to avoid refences to already
deleted items on re-compilation of failed trigger's statememt.
Referencing to already deleted items could take place on re-parsing
a trigger's statement since every Item created for a statement
being re-parsed is deleted before the statement be re-parsed,
but deleted items are still referenced from sp_head. So, to avoid
access to dangling references a per statement list of Item_trigger_field
objects are cleared right after the current SP statement be cleaned up
and before re-parsing be started.