Talk about Slot migration for BloomFilter

[zncleon]

Currently, kvrocks performs slot migration by converting every batch data to RESP command and then sending it to the destination. But I found it's hard to extract RESP about bf.madd from batch for two reason:

1. The inserted items would not be included in both key and value, and we can't get items from value(like bitmap), it's a irreversible conversion.
2. I also can't assign items to log's args in advance for we can't use different args[i] in different batches.

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I find two way to resolve it:

1. Use slot migration based on raw key value Improve slot migration speed and resource consumption using raw key values #1223, but the feature seems not completed and I don't know the details about it.
2. To support some commands like bf.loadchunk, and bf.madd batch would be converted to this RESP command. The batch's value as the command data.

Which way would work, or have some other ways?

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[1] https://redis.io/commands/bf.loadchunk/

[git-hulk]

I prefer the first solution, but it may need some time to test if this PR works well.