noticed while reviewing JDK-8334670

We currently flush the SSLSocketOutputRecord (via hc.handshakeOutput.flush) after every handshake message. Flushing adds unnecessary overhead: record header, padding, MAC. If TCP_NODELAY is not enabled, it may additionally delay the transmission of subsequent messages.

We should only flush when necessary. Examples of places where flushing is necessary include: after Finished message, after ClientHello, after TLS 1.3 ServerHello / HelloRetryRequest, after TLS 1.2 ServerHelloDone. On the other hand, flushing is not necessary between Certificate and CertificateVerify, for example.