Make SslHandler treat OpenSslEngine specially for maximum performance.

[trustin]

Related: #3299

In SslHandler, we have a couple places that checks if the current engine is OpenSslEngine so that we can invoke some non-standard operations in OpenSslEngine for better performance.

However, the non-standard operations still require SslHandler to pass NIO ByteBuffers. We don't really need to enforce this restriction.

For example, OpenSslEngine could accept ByteBuf and CompositeByteBuf rather than NIO buffers, and thus avoid the unnecessary instantiation of NIO buffers.