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.
Any risk that this has unintended consequences of the supervisor? If the strategy is transient, then having the process leaving with `normal` will not attempt a restart, but this will, for example. If we're okay with it or this has no impact, then move along.