Add sharded jscore txn_override passthroughs that step down shard primaries

We have these for replica sets and we have shard primary step down suites for concurrency tests, but not for jscore or with killing and terminating nodes.

Throwing in the balancer as well could add a full dose of chaos.

Non txn_override jscore passthroughs that step down, kill, and terminate shard primaries may also fall out of this work.