all: replace RWMutex with Mutex in places where RLock is not used (#21622)

core/chain_indexer.go

@@ -94,7 +94,7 @@ type ChainIndexer struct {
 	throttling time.Duration // Disk throttling to prevent a heavy upgrade from hogging resources
 
 	log  log.Logger
-	lock sync.RWMutex
+	lock sync.Mutex
 }
 
 // NewChainIndexer creates a new chain indexer to do background processing on