Haraka version 3

[msimerson]

I'm looking forward to dropping node v4 support because node.green.

We've toyed with Haraka v3 in the past but we managed to weasel in most all the changes without requring a semver major. I'm starting to see Haraka v3 as an odd numbered nodejs release. Go fast. Break stuff. Fix it. Move on.

semver major stuff

• plugin cruft littering ./plugins
• drop node v4 testing (and official support)
  • the doors are open to newer ES2015 features
• jettison vm.runInNewContext
  • it's slow and has been a persistent source of memory leaks for years
  • it's not necessary. We aren't executing user-supplied code over the network, we're running plugin files and/or npm modules the admin installed.
  • it's a monumental PITA when testing
  • "people shouldn't be using vm for things they want to do more often than a typical web page would want to boot up an iframe"
  • it has some goofy issues (and horrible nigh-useless debugging) when working with some newer ES6 code
• a plugin registry
  • primary impetus: discoverability of plugins.
  • drop a Plugins.md file into the root of the repo with links to the plugins. NPM, other GitHub repos, our own native plugins. Invite plugin authors to file a PR and add their plugin to the list.
• for node 2 branch, use semver to nail down the versions of NPM packaged plugins
  • plugins targeting Haraka 3 can use SemVer major 3 and drop node 4 support too. When version 3 is stable, bump it to v4 and release it as our LTS branch, and ambitious hackers can have at v5.
• promises
  • like 'em or not, they're coming.
  • they're the foundation of async await in ES2017, so we may as well get the ball rolling.
  • there's likely plenty to promisify
• TLS cert management TLS certificate directory #1828

[msimerson]

Update. The workflow for maintaining separate v2 and v3 branches is pretty easy right now. Commits still happen on master and I periodically merge master to the versioned branches.

Other the "the great plugin cleanup of 2017," the changes between the branches are still minimal. Since the deprecated plugins don't change, it should stay that way for a while longer.

[msimerson]

Closing. We've been able to merge the most salient bits of this PR without a semver major. I expect that to be true for the remaining portions as well.