RUBY-573 ShardingPoolManager seeds format fix and pool handling in cursor

This pull request addresses the failure of ShardingPoolManager to maintain a list of seeds in 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]]

When the connection health is checked, the mongos collection in the config database of the mongos is queried for a list of mongos hosts and ports.  The format was not being converted to be consistent with the internal representation of hosts and ports.  In other words, the mongos hosts and ports were saved in the ShardingPoolManager as strings, not as pairs:  [["localhost", port], ["localhost", port]]

A support method was added so that an array of host:port strings can be converted to an array of host, port pairs using a single method.

The @pool instance variable was also not been set to nil if there was a connection failure in cursor's send_initial_query.  The @pool was then used again to checkout a socket even though the node was not available.