[Async] Multi Exec on cluster

[vladvildanov]

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Description of change

Async implementation of #3611

[vladvildanov]

@petyaslavova @elena-kolevska ClusterPipeline provides only synchronous API for execute_command method, so because of this limitation the only way I found to run asynchronous code inside of synchronous method is to run it inside of short-living thread with it's own event loop. During testing I didn't find a significant performance impact of this (basic set/get test takes around 15ms on my machine).

I think the reason why API is synchronous is the fact that in case of Pipeline execute_command is just about adding command into queue, but for Transactions we need much more that requires asynchronous code execution. Let me know WDYT about this