Fix OPENDJ-127: Back out OpenDS SDK revision 6648 as it prevents the -M options from working, however doing so seems to significantly impact max throughput (around 20%), probably due to context switching during Future.get() calls.

#### From OpenDS SDK SVN history for revision 6650 #####

Backout xxxrate threading changes introduced in revision 6648 since they prevent the -M (throughput throttling) option from working. Several attempts were made to fix it:

1) Putting the adaptive sleeps inside the ConnectionWorker.operationComplete callback causes the Grizzly worker thread to be blocked which, in turn, delays other incoming results on other connections (and impacts their etime calculations).

2) Instead of doing adaptive sleeps, I tried putting the "delayed" requests into a ScheduledExecuterService, but this just introduces more complexity (e.g. more threads), more context switches, etc. 

At this point, I decided to go back to the original design.

The new threading strategy introduced in revision 6649 seems to more than compensate for the performance benefits made in revision 6648.