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.
Think this warrants a spawn_link or a monitor? What happens if we still fail to start a pool because it's the one that is unreachable? Do we retry or does it eventually just forget about it for good?

Do we need to change playbooks if that happens?