Extract _flush_ssl_engine() and call it also in shutdown()
Extract _flush_ssl_engine() and call it also in shutdown()

This fixes java warning:

"Inbound closed before receiving peer's close_notify: possible
truncation attack?"

(when using Netty, for example)

The consequence of that warning is that TLS session of that connection
is not stored and cannot be reused.

nabla-c0d3/nassl,nabla-c0d3/nassl,nabla-c0d3/nassl