groupcache: populate hotCache based on reported peer QPS

[mertovun]

Summary

getFromPeer currently mirrors a remotely fetched value into the hotCache on a fixed 10% of fetches, chosen at random. This was a placeholder — the code carried a TODO(bradfitz): use res.MinuteQps or something smart — and it is a poor hotness signal:

• it pollutes the hotCache with one-off keys, spending a budget that is deliberately capped at ~1/8 of the cache; and
• it takes ~10 round trips on average before a genuinely hot key is mirrored, which is exactly the owner-side load the hotCache exists to shed.

This change makes the value's owner the authority on hotness, which is the only process that can actually observe global demand for a key it owns.

How

• The owner tracks a per-key request rate using an exponentially weighted moving average (keyStats, half-life of one minute) and reports it in the GetResponse.MinuteQps field — which the wire format already carried but nobody populated.
• A non-owning peer mirrors a key into its hotCache only once the owner reports a rate at or above hotQPS.
• The per-key rate state lives inside the mainCache entry, so it is bounded by cache residency and cleaned up by ordinary eviction; there is no separate map to grow or evict.
• lru.Peek (first commit) lets the rate be read/updated without disturbing LRU recency.

The test-only *rand.Rand hook (added in #175) is replaced with an injectable clock, which TestPeers and the new stats tests use to drive QPS deterministically. TestPeers now exercises the real path: cold peers cause no mirroring, and peers reporting hotQPS cause peer-owned keys to be mirrored and subsequently served from the hotCache with zero peer hits.

Compatibility

No wire-format change; MinuteQps already exists in GetResponse. A peer running older code reports 0 and simply never triggers mirroring, so the change is safe in a mixed-version fleet.

Tests

go build ./..., go vet ./..., and go test -race ./... all pass.

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I'm happy to sign the Google CLA if the CLA bot flags this.

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[mertovun]

I signed it!