Parallel shard load

Loads shards in parallel.

Shards are loaded in parallel both when logplex boots as well as when they are reconfigured in a live system using the `bin/replace_shards`. The system is self-healing now. When a redis shard goes down, logplex will continue to try reconnecting until either the shard comes back or the shard cluster is reconfigured.

The sleep in `logplex_redis_writer` init has been increases slightly to reduce the log spamming during redis downdown.
Is the `self` value a special token that means 'do not notify but also do not pretend to be undefined because this has another special meaning somewhere` ? If so, I'd recommend going for something clearer. If not, why not just use `self()` and a pid, or `undefined`?