8231603: (se) Selector implementations do not need to use cancelledKeys

[Alan Bateman]

This is a change to the Selector and SelectionKey::cancel 
implementations to avoid needing the cancelled-key set. Although 
specified, the cancelled-set key is not exposed in the API (except in 
implementations of AbstractSelector where the implementation has access 
to its own cancelled-key set) so the change should not be observable. 
The change has no impact on alternative implementations of 
AbstractSelector.  While in the area, I replaced the use of 
AtomicBoolean for the open state with a VarHandle.  I also inverted the 
valid flag in SelectionKeyImpl to that its initial value is false and so 
doesn't need to be initialized to true with a volatile write.

http://cr.openjdk.java.net/~alanb/8231603/webrev/index.html

-Alan