Also fixed test cases related to this

sql/mysql_priv.h:
  Change default number of open tables in cache and table definition cache size to 1024
storage/innobase/handler/ha_innodb.cc:
  Remove limitation in InnoDB by default on only 8 internal threads

SMP patch
Rate IO patch
Multiple IO threads patch

Problem: <=> operator may return wrong results 
comparing NULL and a DATE/DATETIME/TIME value.

Fix: properly check NULLs.


mysql-test/r/type_datetime.result:
  Fix for bug#37526: asymertic operator <=> in trigger
    - test result.
mysql-test/t/type_datetime.test:
  Fix for bug#37526: asymertic operator <=> in trigger
    - test case.
sql/item_cmpfunc.cc:
  Fix for bug#37526: asymertic operator <=> in trigger
    - if is_nulls_eq is TRUE Arg_comparator::compare_datetime() 
  should return 1 only if both arguments are NULL.