Improve connection limit of coordinator and fetch leader

[th...@126.com]

Hi,
Since the connections of coordinator and fetch leader on the same broker are two individual connections，when connection count reaching the limit, such situation may happen: connection to coordinator and rebalance can be completed but fetching cannot be completed due to connection limit.


For more detailed information, please refer to [KAFKA-14189]   https://issues.apache.org/jira/browse/KAFKA-14189


So Im wondering if we can do some improvement, making the connection limit more flexible, such as allowing extra related connections of a consumer when the connection count limit has been reached?