InnoDB now has background threads to do purge. They were introduced because 
the master thread was doing too many things already and purge was being 
delayed due to insert buffer merge, flushing etc. 

It is also cumbersome to control purge behaviour if purge is done via the
master thread. We need to stop and resume purge for WL#5522.
      
The default setting of innodb_purge_threads will change from 0 to 1.
      
rb://905 Approved by Inaam Rana.