Unnecessary on("error") handlers

[KlausVii]

The adapter constructor always adds an error to the redis clients here:

socket.io-redis-adapter/lib/index.ts

Line 193 in cdb5535

this.pubClient.on("error", this.friendlyErrorHandler);

The handler just logs a warning if there are no other error handlers it

      if (this.listenerCount("error") === 1) {
        console.warn("missing 'error' handler on this Redis client");
      }

Since this constructor is called every time a namespace is created in socket io, we end up adding multiple copies of this handler to the underlying redis client possibly triggering a MaxListenersExceededWarning.

Would it be possible to add these handlers conditionally only if the client does not have any already?