topo: Do not keep txids in memory indefinitely

I mistakenly assumed the block would be freed after processing completed. That
is not true since chaintopology keeps headers and stubs around for reorgs. So
we need to remove the precomputed txids along with the full_txs.

lightningd/chaintopology.c

@@ -108,6 +108,7 @@ static void filter_block_txs(struct chain_topology *topo, struct block *b)
 		txwatch_inform(topo, &txid, tx);
 	}
 	b->full_txs = tal_free(b->full_txs);
+	b->txids = tal_free(b->txids);
 }
 
 size_t get_tx_depth(const struct chain_topology *topo,