perf: convert to string once in setCacheValue (#9992)

## Description

So we don't burn un-necessary CPU in case of dirty store/delete.

```
name                          old time/op    new time/op    delta
CacheKVStoreIterator500-8       23.3µs ± 2%    23.4µs ± 1%    ~     (p=0.151 n=5+5)
CacheKVStoreIterator1000-8      47.0µs ± 2%    46.3µs ± 0%  -1.50%  (p=0.008 n=5+5)
CacheKVStoreIterator10000-8      462µs ± 3%     458µs ± 1%    ~     (p=0.690 n=5+5)
CacheKVStoreIterator50000-8     2.63ms ± 5%    2.51ms ± 2%  -4.63%  (p=0.032 n=5+5)
CacheKVStoreIterator100000-8    8.09ms ±21%    6.98ms ± 4%    ~     (p=0.151 n=5+5)
CacheKVStoreGetNoKeyFound-8      393ns ± 2%     397ns ± 2%    ~     (p=0.421 n=5+5)
CacheKVStoreGetKeyFound-8        269ns ± 5%     268ns ± 3%    ~     (p=1.000 n=5+5)
```

Fixes #9991

---

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store/cachekv/store.go

@@ -238,12 +238,13 @@ func (store *Store) dirtyItems(start, end []byte) {
 
 // Only entrypoint to mutate store.cache.
 func (store *Store) setCacheValue(key, value []byte, deleted bool, dirty bool) {
-	store.cache[conv.UnsafeBytesToStr(key)] = &cValue{
+	keyStr := conv.UnsafeBytesToStr(key)
+	store.cache[keyStr] = &cValue{
 		value:   value,
 		deleted: deleted,
 		dirty:   dirty,
 	}
 	if dirty {
-		store.unsortedCache[conv.UnsafeBytesToStr(key)] = struct{}{}
+		store.unsortedCache[keyStr] = struct{}{}
 	}
 }