Make socket reactor more easy to use

[JackyWoo]

I use SocketReactor in Poco. And I found that user must do resource management in UDF Handler which can be improved by handling it in SocketReactor.

Resource management means:

1. remove all events in NObserver
2. delete UDF Handler

In my case there is a uncaught error in my UDF Handler and it lead to 100% CPU usage.

Possible fix:

1. Clear events SocketReactor::dispatch reveive error.
2. Create a new NObserver whose _pObject is a RefCountedObject

This change may lead to compatibility issue.

[aleks-f]

Please make a SSCCE as a demonstration of the problem, and make a pull request as a fix proposal.

[JackyWoo]

SSCCE ：

Use Net/samples/EchoServer/src/EchoServer.cpp just modify onSocketWritable which throw something.

        void onSocketWritable(const AutoPtr<WritableNotification>& pNf)
	{
		try
		{
			_socket.sendBytes(_fifoOut);
			throw 1;
		}
		catch (Poco::Exception& exc)
		{
			Application& app = Application::instance();
			app.logger().log(exc);
			delete this;
		}
	}

[JackyWoo]

@aleks-f How about the idea Make socket reactor more easy to use? Expecting for reply.

[aleks-f]

It's quite a big diff, did not have time to look at it yet. There are conflicts in the pull now, fix and I'll review as soon as I get a chance.

[JackyWoo]

@aleks-f Conflicts are fixed. Pls review when you have a chance.