Use hash for Block conflict_target

As the consensus block and zero or more non-consensus (uncle) blocks can
have the same number now, number is no longer unique and not a good
conflict_target, so switch to hash.

apps/explorer/lib/explorer/chain/import.ex

@@ -724,7 +724,7 @@ defmodule Explorer.Chain.Import do
     {:ok, blocks} =
       insert_changes_list(
         ordered_changes_list,
-        conflict_target: :number,
+        conflict_target: :hash,
         on_conflict: :replace_all,
         for: Block,
         returning: true,