Add missing OffersMessageHandler::best_block updating

It seems we forgot to ensure `OffersMessageHandler::best_block` is
consistently updated, leading to us building invalid blinded
payment paths for short-lived payment paths after two weeks without
restart.

Author: TheBlueMatt

lightning/src/offers/flow.rs

@@ -183,10 +183,12 @@ impl<MR: MessageRouter, L: Logger> OffersMessageFlow<MR, L> {
 	///
 	/// Must be called whenever a new chain tip becomes available. May be skipped
 	/// for intermediary blocks.
-	pub fn best_block_updated(&self, header: &Header, _height: u32) {
+	pub fn best_block_updated(&self, header: &Header, height: u32) {
 		let timestamp = &self.highest_seen_timestamp;
 		let block_time = header.time as usize;
 
+		*self.best_block.write().unwrap() = BestBlock::new(header.block_hash(), height);
+
 		loop {
 			// Update timestamp to be the max of its current value and the block
 			// timestamp. This should keep us close to the current time without relying on