readStream mapping to other type of readStream to support pipe

[wang007]

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readStream mapping to other type of readStream to support pipe

Describe the feature

ReadStream<Integer> rs; WriteStream<String> ws;

when ReadStream of sub type and WriteStream of sub type do not match, Pipe cannot be performed. so add map function to support this;

rs.map(Integer::toString).pipeTo(ws);

[wang007]

#4126