mode

This is a modification of serg's and guilhem's suggestion in the bug report,
in that it also causes the transaction log to be written to disc.


mysql-test/r/bdb.result:
  Add result.
mysql-test/t/bdb.test:
  Add test
sql/log.cc:
  Create a log-committing event that itself won't be written to the log when 
  we're in autocommit mode.
sql/log_event.cc:
  Add a new subclass of Query_log_event that doesn't write itself to the log, for
  cases where we only want to flush out the transaction and not also write about
  this event.
sql/log_event.h:
  Add a new subclass of Query_log_event that doesn't write itself to the log, for
  cases where we only want to flush out the transaction and not also write about
  this event.