microsecond(TIME)
  alter table datetime<->datetime(6)
  max(TIME), mix(TIME) 

mysql-test/t/func_if.test:
  fix the test case of avoid overflow
sql/field.cc:
  don't use make_date() and make_time()
sql/field.h:
  correct eq_def() for temporal fields
sql/item.cc:
  move datetime caching from Item_cache_int
  to Item_cache_temporal
sql/item.h:
  move datetime caching from Item_cache_int
  to Item_cache_temporal
sql/item_func.cc:
  use existing helper methods, don't duplicate
sql/item_sum.cc:
  argument cache must use argument's cmp_type, not result_type.
sql/item_timefunc.cc:
  use existing methods, don't tuplicate.
  remove unused function.
  fix micorseconds() to support TIME argument
sql/mysql_priv.h:
  dead code
sql/time.cc:
  dead code