To turn the Causal Consistency on in mongo shell need to
1. implement isCausal() function that returns true by default if its connected to >= 3.6 wire version.

Mongo.prototype.isCausal = function() {
    if (typeof this._isCausal !== 'undefined') {
       return this._isCausal;
    }
    return (this.getMinWireVersion() <= 6 && 6 <= this.getMaxWireVersion());  
}

2. always use isCausal() instead of this._isCauasl