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.
Agreed, this should be improved. In its current form, logplex will boot, attempt to connect and fail. It will complain (the readers silently fail, the writers will make several attempts at connecting before giving up) via logging, and start ignoring that failed shard. Running bin/replace_shards is required to fully repair the situation.

I see two options:

1. Continue with current theme, assuming that it is more important to get a partially functioning logplex cluster running.
2. Add a spawn_link or monitor and make repeated attempts to reconnect to the redis-shard, assuming that the shard can be repaired and no logplex configuration change is required.