silvertip: Fix race condition in 'dispatchMessages'

'SelectionKey#isValid' returns false if a selection key is cancelled,
but if the selection key is cancelled, 'SelectionKey#isAcceptable',
for example, throws a CancelledKeyException. Fix the method so that
exceptions are properly handled.

Signed-off-by: Jussi Virtanen <jussi.virtanen@valotrading.com>