src,stream: improve DoWrite() and Write()

StreamBase::Write() calls StreamResource::DoTryWrite() and StreamResource::DoWrite() to do the job. However the interface definition of StreamResource::DoWrite() is error-prone or at least confusing, which I guess leads to an wrong(but no effect fortunately) override, Http2Stream::DoWrite(). This PR takes care of these two issues and make the StreamBase::WriteString() more efficent in the case where the underlying stream is too busy to receive more data.