Consider the scenario:
1. A client sends an isMaster request to the server over a specific horizon h.
2. A reconfig happens, making horizon h no longer valid.
3a. In the non-exhaust case, the next isMaster request will fail to find the host.
3b. In the exhaust case, the connection will still be live because the connection is still open.

We will respond with an error to all waiting isMasters for reconfigs that change the horizon. This terminates the exhaust stream, when exhaust is used.