(VDB-570) Handle duplicate storage diffs

- If processing a new diff for a row that already exists in the DB,
  ignore the error without logging or queueing
- If processing a queued diff for a row that already exists, remove
  it from the queue

libraries/shared/storage/utils/errors.go

@@ -17,9 +17,12 @@
 package utils
 
 import (
+	"errors"
 	"fmt"
 )
 
+var ErrRowExists = errors.New("parsed row for storage diff already exists")
+
 type ErrContractNotFound struct {
 	Contract string
 }