enforce asynchrony in 'gethistoryOffset'

lib/hk-util.js

@@ -436,7 +436,8 @@ const storeMessage = function (Env, channel, msg, isCp, optionalMessageHash) {
       * -1 if you didn't find it
 
 */
-const getHistoryOffset = (Env, channelName, lastKnownHash, cb) => {
+const getHistoryOffset = (Env, channelName, lastKnownHash, _cb) => {
+    const cb = Util.once(Util.mkAsync(_cb));
     const store = Env.store;
     const Log = Env.Log;