RUBY-573 ShardingPoolManager seeds format fix

This pull request addresses the failure of ShardingPoolManager to convert the format of seeds passed as an argument on initialization into the correct format.

The seeds are passed in as an array in the format ["localhost:port", "localhost:port"]
It should be converted to the format [["localhost", port], ["localhost", port]] but it wasn't being done.

cache_discovered_seeds should be called on initialization of ShardingPoolManager, as it is in the initialization of PoolManager.