Integrate an unnecessary local variable into an instance variable
When analyzing `shards.yml`, structurally detect slave groups instead of doing so by matching keys
Make `send_queries_to_*` methods a bit more DRY
Make `run_queries_on_shard` and `send_queries_to_slave` methods a bit more DRY
Make `should_send_queries_to_*` methods a bit more DRY