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.
It was decided that this issue will be tackled in a another refactoring effort.