wallet: store last block number we searched for UTXOs.

We already go back 100 from this in case of reorgs, so the block number
itself is sufficient.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

lightningd/chaintopology.c

@@ -348,6 +348,10 @@ static void updates_complete(struct chain_topology *topo)
 		/* Maybe need to rebroadcast. */
 		rebroadcast_txs(topo, NULL);
 
+		/* We've processed these UTXOs */
+		db_set_intvar(topo->bitcoind->ld->wallet->db,
+			      "last_processed_block", topo->tip->height);
+
 		topo->prev_tip = topo->tip;
 	}