DAOS: zero-copy data-path counters + cold-buffer warmth guard

[eisenhauer]

Two small additions to DaosWriter, both around the existing MinDeferredSize zero-copy test:

• Counters — per-rank zero-copy vs copy Put split, emitted as DAOS_ZeroCopy*/DAOS_Copy* timers in profiling.json. Visibility into how much of a workload actually zero-copies.
• Warmth guard — forces a large buffer that has never recurred to copy rather than zero-copy, since a cold (unregistered) zero-copy loses to a memcpy into the already-registered chunk. Reuse is tracked by address over a short step window. On by default (DAOS_WARMTH_WINDOW=8, 0 disables). Copy to a warm buffer is better than a cold zero-copy put(), so we only try zero-copy Put() if we've seen that user address before.