src: make SecureContext fields private

[tniessen]

These fields should not be public. Only ctx_ is used outside of the class itself, and should be accessed through the ctx() function instead.

[nodejs-github-bot]

Review requested:

• @nodejs/crypto

[bnoordhuis]

That ssl_ctx() function was only used once until now though. :-)

IMO, ssl_ctx() is kind of bad in that it hides the fact that the SSL_CTX is stored in a smart pointer; it obscures its lifetime. It'd be clearer to have:

const SSLCtxPointer* ctx() const{return &ctx_} // call as: sc->ctx()->get()

(Insert not-a-problem-in-Rust comment)

[tniessen]

@bnoordhuis I changed it to return a const reference to the smart pointer instead :)

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/44128/

[nodejs-github-bot]

Landed in 9a3326f