wallet: avoid rescans if under the snapshot

Refuse to load a wallet if it requires a rescan lower than the height of
an unvalidated snapshot we're running -- in more general terms, if we
don't have data for the blocks.

src/node/interfaces.cpp

@@ -723,6 +723,11 @@ public:
             notifications.transactionAddedToMempool(entry.GetSharedTx(), 0 /* mempool_sequence */);
         }
     }
+    bool hasAssumedValidChain() override
+    {
+        return Assert(m_node.chainman)->IsSnapshotActive();
+    }
+
     NodeContext& m_node;
 };
 } // namespace