Add host status event handlers to ILoadBalancingPolicy interface

Description

The load balancing policy should be responsible for handling node status changed events and triggering a distance changed event if necessary. Currently the driver "polls" all LBPs on every request and when the CC receives status changed events to determine whether the host distance has changed.

See java driver's LBP interface for an example of this:

https://github.com/datastax/java-driver/blob/9f0d89799a8a1e4cd1022dd7c43333924c36a648/core/src/main/java/com/datastax/oss/driver/api/core/loadbalancing/LoadBalancingPolicy.java

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Reporter

Joao Reis

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