Major Description
If the client performs a snapshot scan, we may need to wait for the leader to tell us that the timestamp is "safe". If the majority of nodes in a tablet are down, this will never happen. After KUDU-689, well wait with a deadline, but even this multi-second wait will end up blocking a lot of RPC handlers, potentially preventing other useful work from getting done.
We should probably short-circuit the wait in the case that we haven't heard from any leader within the election timeout and just respond immediately. Alternatively, we could make this an async callback vs a blocking wait on handler.