PIP-122: Change loadBalancer default loadSheddingStrategy to ThresholdShedder

The ThresholdShedder load balance policy since Pulsar 2.6.0 by https://github.com/apache/pulsar/pull/6772. It can resolve many load balance issues of OverloadShedder and works well in many Pulsar production clusters.

In Pulsar 2.6.0, 2.7.0, 2.8.0 and 2.9.0, Pulsar's default load balance policy is OverloadShedder.

I think it's a good time for 2.10 to change default load balance policy to ThresholdShedder, it will make throughput more balance between brokers.

Proposed Changes

In 2.10 release，for broker.conf, change loadBalancerLoadSheddingStrategy from org.apache.pulsar.broker.loadbalance.impl.OverloadShedder to org.apache.pulsar.broker.loadbalance.impl.ThresholdShedder