Memory store repartitioning behavior

[Roman1us]

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Description

I use memory store on production for aggregation tables. So, if we are use 2 partitions on table's changelog topic, we want table keys to be partitioned accross workers. But, in memory store we will have old keys, that, after repartition (rebalance), belongs to new worker. In some cases, when we modify table from stream, it won't be a problem, but i use use_partitioner and modify table by task, and for me this is a big problem.

So i add extra logic for memory store, that stores partitions for keys. Also added clearing no longer used keys on the worker after recovery from table.

[codecov-commenter]

Codecov Report

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The diff coverage is 100.00%.

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@@            Coverage Diff            @@
##             master     #199   +/-   ##
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  Coverage          ?   94.46%           
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  Files             ?      100           
  Lines             ?    10665           
  Branches          ?     1206           
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  Hits              ?    10075           
  Misses            ?      525           
  Partials          ?       65           

Impacted Files	Coverage Δ
faust/stores/memory.py	100.00% <100.00%> (ø)

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