Remove `socket` server from ActionHero core; move to a plugin

[evantahler]

• ActionHero Version: 20.x.x (will be a breaking change)
• Node.js Version: all
• Operating System: all

Usage for the 'socket' server in Actionhero is low (slack thread: https://actionherojs.slack.com/archives/C04EVSUSD/p1567476416040700). In order to keep the core of Actionhero small, we should remove the socket server from the core project into a plugin. This would require moving:

• The server
• The tests
• The docs

We would also need a good way to host the documentation for the plugin (is the README enough?)

[evantahler]

closing in favor of PR https://github.com/actionhero/actionhero/pull/1349