nsIAsyncStreamCopier in Firefox 1.5

 asyncCopy triggers the start of the copy.  The observer will be notified
 when the copy completes.

 @param aObserver
        receives notifications.
 @param aObserverContext
        passed to observer methods.

 Initialize the stream copier.

 @param aSource
        contains the data to be copied.
 @param aSink
        specifies the destination for the data.
 @param aTarget
        specifies the thread on which the copy will occur.  a null value
        is permitted and will cause the copy to occur on an unspecified
        background thread.
 @param aSourceBuffered
        true if aSource implements ReadSegments.
 @param aSinkBuffered
        true if aSink implements WriteSegments.
 @param aChunkSize
        specifies how many bytes to read/write at a time.  this controls
        the granularity of the copying.  it should match the segment size
        of the "buffered" streams involved.

 NOTE: at least one of the streams must be buffered.