A shard can add additional shard as a participant for a transaction and when it does so

1. It lets the router know
2. It sends the following message from itself to the added shard

{     insert: “mycollection”,     documents: [ \{ _id: 1 }

, { _id: 2 } ],

    lsid: { id: UUID(...) },

    txnNumber: NumberLong(2),

    autocommit: false,

    startOrContinueTransaction: true,

}

Update the incoming mongod interface to support this.