Add weights to server side connection throttling

In ZOOKEEPER-3242, we introduced connection throttling to protect the server from being overloaded. We realize that the costs for creating a local session, creating a global session, and reconnecting are different. So we associate weights to the costs when throttling. For example, for the same setting, the throttler will allow more connections to be created if they are local.  This allows the server resources to be fully utilized.