permit redis pub/sub to work when host != localhost

[msimerson]

it's complicated

Previously, the redis plugin worked well, except when the pub/sub channel was on a host other than 127.0.0.1. PR #1275 sorta fixed the karma plugin so that it could subscribe to a remote redis when the redis plugin (and by extension, result_store) could pub/sub to that remote redis host. It was not an optimal way to fix it though, because it duplicated the redis.ini parsing and connection logic.

With these changes, the redis connection and subscription functions are consolidated into the redis plugin, where they can be called by plugins that subscribe to result_store messages (currently karma & watch). Now the redis plugin has a config section for pubsub and all publishers and subscribers can use the same settings, without each subscriber having to duplicate the pubsub connection logic. That logic was complicated by the fact that subscribers must have a different redis client than the publisher.

the changes

• redis.md: add Publish/Subscribe usage
• redis.js:
  • rename plugin.cfg -> plugin.redisCfg (make it safe to inherit redis.js)
  • refactored get_redis_client() so it can be reused
  • add channel naming logic consistent with result_store publishing (so subscribers don't have to specify)
  • cleaned up connection log messages (see below diff)
    • client.host and client.port aren't always defined
• karma.ini: rename redis config settings to match redis.ini (with backwards compatible shims)
• karma.js:
  • inherit redis
  • remove load_redis_ini()
  • move config defaults into config loading function
  • use inherited redis pub/sub for sub connection
  • use new redis.get_redis_client for getting client
• watch:
  • newer version that uses pub/sub instead of traversing connection object
  • inherits redis.js (so it too can subscribe to a remote redis)

connection message

-Dec 23 14:12:00 node haraka[10834]: [INFO] [-] [redis] connected to undefined/3 v3.0.4
-Dec 23 14:59:12 node haraka[50398]: [INFO] [-] [redis] dbid 3 selected
+Dec 23 17:29:15 node haraka[84229]: [INFO] [-] [redis] connected to redis://172.16.15.16:6379 v3.0.4