Chunk dedupe bug

[mhambre]

Replaying this fails occasionally (sometimes after 100+ attempts, sometimes after only a few). It's not pressing yet because it's just an optimization, but since we're going through and adding a lot of changes I'm not too pressed as we may end up fixing it anyways:

i=1
while cargo test same_offset_concurrency_dedupe_shares_one_upstream_fetch --features utils,http; do
  echo "pass #$i"
  i=$((i+1))
done

echo "failed on iteration #$i"

There's the possibility that it's a test race since we don't have probes in the oracle to force block so it's contingent on proper timing.