mollifier:envs is a Redis SET that grows with the count of envs that
currently have buffered entries. Under normal operation that's small,
but an extended drainer outage can leave entries piled up across
thousands of envs — at which point runOnce would queue one
processOneFromEnv per env through pLimit, ballooning per-tick latency
and event-loop queue depth.
Cap per-tick fan-out at MOLLIFIER_DRAIN_MAX_ENVS_PER_TICK (default 500).
When the set fits within the cap, behaviour is unchanged (take all,
rotate cursor by 1 for fairness). When the set exceeds the cap, take a
rotating slice and advance the cursor by the slice size so successive
ticks sweep through the full set.
Tests use a stub buffer to drive listEnvs() deterministically with
thousands of envs without provisioning a real Redis.
Dan Sutton
f91cbf2b2a