Is your feature request related to a problem? Please describe.

As we ship OpenSSL on Windows, we have to update Icinga along with OpenSSL.
Every OpenSSL update is a security update by definition.

Describe the solution you'd like

• typedef boost::asio::ssl::stream SslStream; or similar
• use SslStream everywhere, not boost::asio::ssl::stream
• on Windows make SslStream a custom class using SChannel, not OpenSSL
• same with boost::asio::ssl::context => SCHANNEL_CRED

Describe alternatives you've considered

• [Feature] Update OpenSSL bundled with Icinga 2 independent of Icinga 2 itself icinga-powershell-framework#674 (IfW only)