update latest block query

the format of the settlements table will be changed.
instead of looking at the earlies block without some transaction data
we now look at the latest block with settlement observations

this should work with both the new and old format of the settlements table

src/sql/orderbook/latest_block.sql

@@ -1,3 +1,2 @@
-select min(block_number) latest
-from settlements
-where tx_from is null;
+select max(block_number) latest
+from settlement_observations;