8212132: (dc) Remove DatagramChannelImpl finalize method

[Alan Bateman]

Lots of good work in recent releases to replace the use of finalizers in 
the java.base module with the cleaner mechanism. The DatagramChannel 
implementation is still using a finalizer so it's time to replace that. 
While in the area I removed the wrapper around the cleaner used in the 
NIO SocketImpl as it is not needed.

The proposed changes are here:
   http://cr.openjdk.java.net/~alanb/8212132/webrev/index.html

-Alan