Possible inefficiency on reconnect

[edemaine]

For #365 I've been reading over the reconnect code. If I'm reading https://github.com/cult-of-coders/redis-oplog/blob/master/lib/redis/RedisSubscriptionManager.js correctly:

• attach() may add the same redisSubscriber to this.store[channel] for multiple channels.
• getAllRedisSubscribers() will then get all copies of all redisSubscribers, causing onConnect() to reload some subscribers many times.

Assuming folks agree with me, I'm happy to prepare a PR where getAllRedisSubscribers deduplicates all the redisSubscribers using a Set. But I just wanted to double-check I'm not missing something first.

(In my application, I'm pretty sure each redisSubscriber has only one relevant channel, so this doesn't affect me. But it struck me when reading the code.)