Wait 2s after switching server mode before answering. (#6390)

This is needed because the change is propagated via the agency and the
heartbeat, which only happens once per second.

arangod/Cluster/ServerState.cpp

@@ -794,6 +794,10 @@ Result ServerState::propagateClusterReadOnly(bool mode) {
     if (!r.successful()) {
       return Result(TRI_ERROR_CLUSTER_AGENCY_COMMUNICATION_FAILED, r.errorMessage());
     }
+    // This is propagated to all servers via the heartbeat, which happens
+    // once per second. So to ensure that every server has taken note of
+    // the change, we delay here for 2 seconds.
+    std::this_thread::sleep_for(std::chrono::seconds(2));
   }
   setReadOnly(mode);
   return Result();