XmlScoreBoardServlet update debugGet to print NOT_MODIFIED

Currently debugGet prints out each XML doc event that's sent to
polling clients, this adds a line print for each response to
clients when there is no change
Simplify sleeping queued XML event listener

Much of the listener is duplicated from the regular queued listener
so just subclass that and re-use its methods.  Also change to use
the documentLock's Object monitor wait/notify instead of separately
sleeping and tracking all waiting Threads (that way is racy
since the lock is released non-atomically before actually sleeping)