init the binlog_cache (THD::transaction.trans_log).
I have checked all places where trans_log is used, because as now
it may not be inited in some cases, we have to be cautious
(will forward this commit mail to Heikki).


sql/handler.cc:
  Do not use the transaction binlog cache if binlog is not open
sql/sql_class.cc:
  We do not init the transaction binlog cache if binlog is not open.
  This saves 32 KB memory per thread, if --log-bin is not used.
sql/sql_class.h:
  warning comment