Change to not store records internally in the actor

[vlovgr]

This pull request changes KafkaConsumerActor to not store records internally when records are received for partitions for which there are no requests. Instead, we change to poll(Duration.Zero) when there are no requests. This avoids assignment and records to be returned in one poll, which is the cause for 'records for partitions without requests'. This is also similar to what Alpakka Kafka does, which seems to have worked there. We add checks to make sure nothing unexpected is received.

[codecov]

Codecov Report

Merging #129 into 0.20.x will increase coverage by 0.39%.
The diff coverage is 79.06%.

@@            Coverage Diff             @@
##           0.20.x     #129      +/-   ##
==========================================
+ Coverage   93.35%   93.75%   +0.39%     
==========================================
  Files          40       40              
  Lines        1174     1153      -21     
  Branches       80       78       -2     
==========================================
- Hits         1096     1081      -15     
+ Misses         78       72       -6

Impacted Files	Coverage Δ
src/main/scala/fs2/kafka/internal/LogEntry.scala	25% <ø> (-1.48%)	⬇️
.../scala/fs2/kafka/internal/KafkaConsumerActor.scala	84.5% <79.06%> (+0.88%)	⬆️

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