kafka: avoid reactor stall in legacy message set handling

[dotnwat]

I wonder if this should be futurized (probable a large knock-on effect, so another PR)

Originally posted by @BenPope in #1389 (comment)

The legacy messages handed in produce are optionally uncompressed, repackaged into our internal format, and optionally recompressed. This poses a reactor stall risk, but the code change is a bit involved. If we encounter an issue here we can apply some workarounds to futurize the costly bits.

JIRA Link: CORE-628