outbound: get_mx overhaul, logging improvements

[msimerson]

Fixes #3314

Conceptually, the big changes in this PR do the following:

1. outsource outbound DNS to haraka-net-utils
2. improve outbound logging by using logger in only 2 common ways: logger.info(obj, message) or this.loginfo(self, msg...)
3. In outbound, the control flow is much more linear:
   • run get_mx hooks
   • if no plugin offered a MX, get from net_utils.get_mx (which now includes implicit MX)
   • pass list of MXes to found_mx, which sorts the MXes
   • net_utils resolves all MX hostnames to IPs (IPv4, IPv6, all at once, for speed)
   • each MX IP is tried in turn until one succeeds or the list is exhausted
4. Injecting logging functions into classes (Connection, Plugin, HMailItem) is now done via logger.add_log_methods.

Changes:

• outbound: minor es6 updates
• outbound: logging improvements
  • was: [-] [core] [outbound] Failed to get socket: Outbound connection error: Error: connect ECONNREFUSED 172.16.16.14:25
  • now: [A63B62DF-F...] [outbound] Failed to get socket: connect ECONNREFUSED 172.16.16.14:25
  • outbound: remove log prefixes of [outbound], no longer needed
  • shorter logger syntax: logger.loginfo -> logger.info
• logger(connection, plugins, hmail), refactor copying in log functions
• logger: don't load outbound (race condition). Instead, set name property
• outbound: delete try_deliver_host. Use net_utils to resolve MX hosts to IPs
• outbound: remove config setting ipv6_enabled
• fix(logger): refactor add_log_methods, don't set extra loglog* names
• doc(connection): update rfc7001 URL

Checklist:

• [x] docs updated
• [x] tests updated
• [x] Changes updated