8202252: (aio) Closed AsynchronousSocketChannel keeps completion handler alive

[Brian Burkhalter]

https://bugs.openjdk.java.net/browse/JDK-8202252
http://cr.openjdk.java.net/~bpb/8202252/webrev.00/

Set instance variables {connect,read,write}Handler to null to allow the respective objects to be GCed.

Without applying the source portion of the patch, the test fails on Linux, macOS, and Solaris.

Thanks,

Brian