8223353: (ch) Change channel close implementation to not wait for I/O threads

[Alan Bateman]

I'd like to refactor the SelectableChannel close implementations so that 
the close methods don't wait for I/O operations to abort when closing a 
channel in blocking mode.

As I'm sure we all know here, closing a channel is complicated because 
it requires coordinating with threads doing I/O operations. It also 
requires coordinating with selection operations as the channel may be 
registered with one or more Selectors. As things currently stand, 
closing a channel in blocking mode with threads blocked in I/O 
operations involves:

1. pre-close (the dup2 trick to close the connection but not release the 
file descriptor)
2. signal the threads in blocking I/O operations so that they abort with 
EINTR
3. wait for the threads to complete/abort the I/O operation
4. close the file descriptor (if not registered with a Selector - not 
usually possible with channels configured in blocking mode but there are 
convoluted scenarios to consider).

For comparison, the non-blocking case is a simple lock/unlock to ensure 
that all I/O operations have completed and then closing the file 
descriptor when not registered with a Selector. If registered with a 
Selector then the close is deferred until its key is flushed from the 
last Selector that it is registered with.

The proposed change is to eliminate #3 above and have the last I/O 
operation to complete do the final close. This simplifies the close nd 
means that I/O operation in progress and registered with Selector cases 
are handled consistently. The change should not be observable to code 
using these APIs as the pre-close will close the connection as before.

One benefit of the change is that is Thread.interrupt doesn't wait. A 
second benefit is that we move stateLock in the implementations back to 
being a built-in monitor, the condition object used to await/signal, 
goes away.

The webrev with the proposed changes is here. The changes to stateLock 
are purely mechanical, everything else is the refactor of the 
implCloseSelectableChannel implementations.

http://cr.openjdk.java.net/~alanb/8223353/0/webrev/

-Alan