Update the leader term in ZK on the condition that the replica is still the leader

While working on SOLR-13141, I realized that the ZkShardTerms.ensureTermIsHigher and related methods do a compare-and-set on the terms but there is no guarantee that the leader is still the leader when the zk update executes. This can potentially lead to race conditions during leader transitions.

We should update the term using a zk multi-op conditional on the current replica still being the leader. This will not change any behavior but will only be an additional safety check.