ConcurrentHashMap.put(key, null) throws an NPE effects BaseIOSession attachments/attributes

the pattern to remove an attachment on an IOSession is to do a:

setAttachment(null)

In 1.1 this throws a NPE because the backing map of ConcurrentHashMap throws an exception when one tries to put a null.

This is a problem for both attachments & attributes.

Can fix by:

• add a removeAttachement() call
• special case nulls in setAttachement & setAttribute
• change the backing map.

This causes a problem in the proxy example where the client doesn't get responses back from the server.