perf(state): remove fmt.Sprintf within for-loop (#1790)

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## Overview

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<!-- This is an auto-generated comment: release notes by coderabbit.ai
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## Summary by CodeRabbit


- **Bug Fixes**
- Adjusted the method for constructing composite keys within the
indexing process, which may enhance string matching.
- **Improvements**
- Simplified string concatenation for composite key creation,
potentially improving overall performance.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Co-authored-by: Khanh Hoa <[email protected]>

Author: hoanguyenkh

state/indexer/block/kv/kv.go

@@ -540,7 +540,7 @@ func (idx *BlockerIndexer) indexEvents(batch ds.Txn, events []abci.Event, typ st
 			}
 
 			// index iff the event specified index:true and it's not a reserved event
-			compositeKey := fmt.Sprintf("%s.%s", event.Type, string(attr.Key))
+			compositeKey := event.Type + "." + attr.Key
 			if compositeKey == types.BlockHeightKey {
 				return fmt.Errorf("event type and attribute key \"%s\" is reserved; please use a different key", compositeKey)
 			}

state/txindex/kv/kv.go

@@ -162,7 +162,7 @@ func (txi *TxIndex) indexEvents(result *abci.TxResult, hash []byte, store ds.Txn
 			}
 
 			// index if `index: true` is set
-			compositeTag := fmt.Sprintf("%s.%s", event.Type, string(attr.Key))
+			compositeTag := event.Type + "." + attr.Key
 			if attr.GetIndex() {
 				err := store.Put(txi.ctx, ds.NewKey(keyForEvent(compositeTag, attr.Value, result)), hash)
 				if err != nil {