Fixed the synchronization issue for cancel() operations

Locking operations may affect hudson.remoting.Request::get() handlers
The issue may cause the JENKINS-21932, which seems to be caused by unhandled locks().

Signed-off-by: Oleg Nenashev <o.v.nenashev@gmail.com>
Use notifyAll() instead of notify() inside Request::onCompleted() handler

FindBugs indicates it as a probable issue, which can be the origin of JENKINS-16679

Signed-off-by: Oleg Nenashev <o.v.nenashev@gmail.com>