Allow shutdown during LoadMempool, dump only when necessary

src/validation.cpp

@@ -4076,6 +4076,8 @@ bool LoadMempool(void)
             } else {
                 ++skipped;
             }
+            if (ShutdownRequested())
+                return false;
         }
         std::map<uint256, CAmount> mapDeltas;
         file >> mapDeltas;