message: Don't ever remove the userId from the mapping, we need it.
diff: @@ -533,10 +533,15 @@ IrcHandler.prototype.onMessage = Promise.coroutine(function*(req, server, fromUs
  for (let nick in nicks) {
  const userId = mapping[nick];
  const feature = (yield store.getUserFeatures(userId)).mentions;
  - const disabled = feature === true ||
  + const enabled = feature === true ||
  (feature === undefined && this._mentionMode === "on");
  - if (disabled) {
  + if (!enabled) {
  delete mapping[nick];
  + // We MUST keep the userId in this mapping, because the user
  + // may enable the feature and we need to know which mappings
  + // need recalculating. This nick should hopefully never come
  + // up in the wild.
  + mapping["disabled-matrix-mentions-for-"+nick] = userId;
  }
  }
  this.nickUserIdMapCache.set(`${server.domain}:${channel}`, mapping);
  