ObjectMessage: better marshalling

Currently, ObjectMessage accepts only objects of type SizeStreamable and wraps everything else into ObjectWrapper instances.

This means that primitive types (e.g. "7" or "hello world") will be wrapped, too. The disadvantage of ObjectWrapper is that it performs eager marshalling, creating an unwanted byte[] array before the message is actually marshalled. On the receiver side, another byte[] array will be created before the actual unmarshalling starts.

Solution: accept primitive types (e.g. int, Integer, String, AsciiString, byte[] etc) directly, without wrapping them into an ObjectWrapper.