Fixed control connection crash and "DOWN" event handling

- The std::map reference in ControlStartupQueryPlan became invalidate after
the first connection and a node outage
- A "DOWN" event now causes a host's pool to immediately disconnect and timeout
any pending requests. This has know issues if a control connection becomes partitioned
from the rest of the cluster and falsely reports a majority of nodes as "DOWN". However,
this is a better interim solution than waiting for the connections to timeout.