Streamable class might implement SizeStreamable

Many classes implement Streamable, but also provide a serializedSize() method which returns the number of bytes needed for the serialized form.

Methods like Util.streamableToBuffer() could benefit from this: instead of allocating a 512 byte buffer, which is either too big or needs to be resized later, we can allocate a buffer of the exact size.

We should therefore make all classes implementing Streamable instead implement SizeStreamable.

Code that makes use of this:

public static Buffer streamableToBuffer(Streamable obj) {
        int expected_size=obj instanceof SizeStreamable? 
               ((SizeStreamable)obj).serializedSize() +1 : 512;
        final ByteArrayDataOutputStream out=new ByteArrayDataOutputStream(expected_size);
        try {
            Util.writeStreamable(obj,out);
            return out.getBuffer();
        }
        catch(Exception ex) {
            return null;
        }
    }