Forcibly disconnect the client before reconnecting

The current sentinel client handles redis failures, but does not handle redis failovers. In cases of scheduled maintenance, a failover can be forced by executing `SENTINEL failover mymaster`. The sentinel client then sees the failover and changes `.host` and `.port` and calls `forceReconnectionAttempt`. However, on [line 522 of node_redis/index.js](https://github.com/DocuSignDev/node_redis/blob/master/index.js#L522), a reconnection attempt is only executed if `(!this.stream)`, but since the redis instance is not down, the stream still exists. This produces an `activeMasterClient` with `host` and `port` pointing to the new master, but the stream actually going to the new _slave_.

In order to handle this case (and others, such as network partitions), I forcibly set `activeMasterClient.stream = null` so the forced reconnection attempt happens in _every_ case.