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/interfaces/chain.h

@@ -286,6 +286,9 @@ public:
     //! to be prepared to handle this by ignoring notifications about unknown
     //! removed transactions and already added new transactions.
     virtual void requestMempoolTransactions(Notifications& notifications) = 0;
+
+    //! Return true if an assumed-valid chain is in use.
+    virtual bool hasAssumedValidChain() = 0;
 };
 
 //! Interface to let node manage chain clients (wallets, or maybe tools for