Asyncronously splice the output stream of this to the input stream of stream2, and splice 
            the output stream of stream2 to the input stream of this.
When the operation is finished callback will be called. You can then call splice_async.end to 
            get the result of the operation.
| this | 
                 a IOStream.  | 
            
| stream2 | 
                 a IOStream.  | 
            
| flags | 
                 a set of IOStreamSpliceFlags.  | 
            
| io_priority | 
                 the io priority of the request.  | 
            
| cancellable | 
                 optional Cancellable object, null to ignore.  | 
            
| callback | |
| user_data | 
                 user data passed to   |