Use LibreSSL?

[tengyifei]

OpenSSL is much more prone to vulnerabilities than LibreSSL. Perhaps we can switch to that instead of using an embedded version of OpenSSL? Vapor and Zewo are all using LibreSSL instead of OpenSSL.

Alternatively let the user supply an API-compatible xxxSSL library like what the Swift-NIO team is doing: https://github.com/apple/swift-nio-ssl