KEY UPDATES WITH A LIMIT OF 1

Problem: The unsafety warning for statements such as
update...limit1 where pk=1 are thrown when binlog-format
= STATEMENT,despite of the fact that such statements are
actually safe. this leads to filling up of the disk space 
with false warnings.
 
Solution: This is not a complete fix for the problem, but
prevents the disks from getting filled up. This should
therefore be regarded as a workaround. In the future this
should be overriden by server general suppress/filtering
framework. It should also be noted that another worklog is
supposed to defeat this case's artificial unsafety.

We use a warning suppression mechanism to detect warning flood,
enable the suppression, and disable this when the average
warnings/second has reduced to acceptable limits.
 
  Activation: The supression for LIMIT unsafe statements are
  activated when the last 50 warnings were logged in less 
  than 50 seconds. 
 
  Supression: Once activated this supression will prevent the
  individual warnings to be logged in the error log, but print
  the warning for every 50 warnings with the note:
  "The last warning was repeated N times in last S seconds"  
  Noteworthy is the fact that this supression works only on the
  error logs and the warnings seen by the clients will remain as
  it is (i.e. one warning/ unsafe statement)
 
  Deactivation: The supression will be deactivated once the
  average # of warnings/sec have gone down to the acceptable limits.



sql/sql_class.cc:
  Added code to supress warning while logging them to error-log.