Adapt per-peer pipeline window based on relative lag - #658
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Replace the flat max_pipeline_count cap with a per-peer window that grows toward the ceiling when a peer falls behind its fastest cluster-mate, and shrinks toward a batching floor (512) when it's keeping pace. Comparing lag against the fastest peer rather than the log tip avoids false growth during healthy write bursts, since a burst advances every peer's reference point equally - only a peer genuinely lagging its cluster-mates triggers growth. Any append_entries_reply failure, snapshot backoff, or snapshot install completion resets a peer's window straight back to the ceiling, so a struggling or recovering peer always gets the safe, generous default rather than trusting a previously-shrunk value.
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Replace the flat max_pipeline_count cap with a per-peer window that grows toward the ceiling when a peer falls behind its fastest cluster-mate, and shrinks toward a batching floor (512) when it's keeping pace. Comparing lag against the fastest peer rather than the log tip avoids false growth during healthy write bursts, since a burst advances every peer's reference point equally - only a peer genuinely lagging its cluster-mates triggers growth.
Any append_entries_reply failure, snapshot backoff, or snapshot install completion resets a peer's window straight back to the ceiling, so a struggling or recovering peer always gets the safe, generous default rather than trusting a previously-shrunk value.