Fix read-only query latency for large heartbeat intervals

Read-only queries wait for a round of heartbeats to come back from a
majority of the cluster. They were meant to schedule a round of
heartbeats right away, but the code was missing a broadcast to a
condition variable, causing the read-only queries to wait for the
regularly scheduled heartbeats. This isn't so terrible with the default
heartbeat interval of 75ms, but it's not ok for higher intervals.

Server/RaftConsensus.cc

@@ -2346,6 +2346,7 @@ RaftConsensus::upToDateLeader(std::unique_lock<Mutex>& lockGuard) const
     uint64_t epoch = currentEpoch;
     // schedule a heartbeat now so that this returns quickly
     configuration->forEach(&Server::scheduleHeartbeat);
+    stateChanged.notify_all();
     while (true) {
         if (exiting || state != State::LEADER)
             return false;

Server/RaftConsensus.h

@@ -1302,6 +1302,7 @@ class RaftConsensus {
      *  - startElectionAt changes (see note under startElectionAt).
      *  - an acknowledgement from a peer is received.
      *  - a server goes from not caught up to caught up.
+     *  - a heartbeat is scheduled.
      * TODO(ongaro): Should there be multiple condition variables? This one is
      * used by a lot of threads for a lot of different conditions.
      */