Polyfill for keylog API

[mildsunrise]

This is not (at least initially) intended to be merged, just as a proposal.
Basically implements a polyfill that:

• for Node.JS >= 11.9.0, calls SSL_CTX_set_keylog_callback natively and emits keylog events based on it
• for Node.JS < 11.9.0, emulates the OpenSSL API by manually assembling the CLIENT_RANDOM line as we do now, and emitting keylog events.
• for Node.JS >= v12.3.0 (probably), if it detects that keylog is available natively, does nothing

Polyfilling the keylog API would allow us to implement index.js without needing the native module:

require('./lib/pollyfill.js');

E.filename = // ... (no changes)
E.set_log = // ... (no changes)
E.log_line = line => fs.appendFile(E.filename, line, err => {
    if (err) console.error(`Warning: Failed to log to SSLKEYLOGFILE: ${err}`);
});

E.hook_socket = socket => socket.on('keylog', E.log_line);
E.hook_server = server => server.on('keylog', E.log_line);
E.hook_agent = // ... (no changes)
E.hook_all = // ... (no changes)

Which would in turn allow us to skip building the native module entirely, if the native API is present. So, for recent Node.JS versions, this would simply be a small JS module for easy SSLKEYLOGFILE logging.

The downside is that we'd have to drop update_log and get_session_key. If someone needs custom logging he can subscribe to keylog directly, and only use this module as a polyfill. The second one is not entirely replaceable, but you can parse keylog lines to get the secrets you need (with TLSv1.3 this should be even more reliable than calling get_session_key manually, IMHO). What are your thoughts on this?

[kolontsov]

Hi Alba, sorry for pause, busy days.. Everything sounds logical, and you're right - advantages are clear. Let's move along the way you've suggested, and we'll see how it will work out. I'll add more comments after reading proposed code.

[mildsunrise]

No worries, and thanks for your time ^^

[kolontsov]

Hi, long time no see! I reviewed all the code (and thanks for tests!), and I'd like to merge it to move forward. What do you think?

[mildsunrise]

Hi! Seems good to me, I've been using it for a while, it works well