kv: run flushRawPending in goroutine to avoid blocking the first caller

The first commitRaw caller that becomes the flusher was blocked
synchronously in the flush loop, potentially processing unrelated
batches long after its own result was ready. Running the flush in
a goroutine lets each caller block only on its own done channel.

Author: bootjp

kv/transaction.go

@@ -164,7 +164,7 @@ func (t *TransactionManager) commitRaw(reqs []*pb.Request) (*TransactionResponse
 	t.mu.Unlock()
 
 	if shouldFlush {
-		t.flushRawPending()
+		go t.flushRawPending()
 	}
 
 	res := <-item.done