Record generation_id after fetch

[ekerstens]

Description

Limiting the comparison of generation_id to after the fetcher returns limits the scope of the check to a change of generation_id within getmany itself, leaving it up to the fetcher itself to avoid returning records after a rebalance. This avoids an issue where a rebalance happens during fetching, fetching waits for the rebalance to finishes to return, but then the fetch is discarded (incorrectly) by getmany.

[codecov-commenter]

Codecov Report

Merging #235 (eaa6c28) into master (40e477f) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #235   +/-   ##
=======================================
  Coverage   94.33%   94.33%           
=======================================
  Files         100      100           
  Lines       10759    10759           
  Branches     1471     1471           
=======================================
  Hits        10149    10149           
  Misses        542      542           
  Partials       68       68           

Impacted Files	Coverage Δ
faust/transport/consumer.py	95.28% <100.00%> (ø)

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