Fix a race: RateController::CustomFetch::HandleDone could call DeleteFetchInfoIfPossible and have it destroy the map entry and HostFetchInfoPtr + HostFetchInfo simultaneously with a new call to ::Fetch having grabbed the entries and released RateController::mutex_ but not yet incremented the outstanding events count.
Fix a race: RateController::CustomFetch::HandleDone could
call DeleteFetchInfoIfPossible and have it destroy the map
entry and HostFetchInfoPtr + HostFetchInfo simultaneously with
a new call to ::Fetch having grabbed the entries and released
RateController::mutex_ but not yet incremented the outstanding
events count.

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