size computation in SkipOp

[Arne Siegel]

I'd expect the following computation in SkipOp.wrapSink() makes some sense:

        return new Sink.ChainedValue<T>(sink) {
...
            @Override
            public void begin(int size) {
                downstream.begin(size < 0 ? size : size >= skip ? size - skip : 0);
            }

If stream size gets computed in this way, FLAG_SIZED in SkipOp.getStreamFlags() doesn't 
need to be cleared.

Similarly for MapSkipOp.

Regards
Arne Siegel