Cancel getmany if generation_id changes

[ekerstens]

Description

Found one more edge case where events from before a rebalance can leak through. Consumer.getmany is designed to break out of the loop during a rebalance. However, if the asyncio loop never returns control to getmany during the rebalance, it will continue to yield events from before the rebalance, and on_message assigns the incorrect generation_id` Adding logic to check if the generation_id has changed for more robust handling.

[codecov-commenter]

Codecov Report

Merging #234 (e4b1391) into master (d9533ca) will decrease coverage by 0.02%.
The diff coverage is 40.00%.

@@            Coverage Diff             @@
##           master     #234      +/-   ##
==========================================
- Coverage   94.35%   94.33%   -0.03%     
==========================================
  Files         100      100              
  Lines       10754    10759       +5     
  Branches     1470     1471       +1     
==========================================
+ Hits        10147    10149       +2     
- Misses        540      542       +2     
- Partials       67       68       +1     

Impacted Files	Coverage Δ
faust/utils/json.py	100.00% <ø> (ø)
faust/transport/consumer.py	95.28% <40.00%> (-0.51%)	⬇️

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