remove stale mempool txs before fetching new ones (and possibly failing)

src/new_index/mempool.rs

@@ -510,6 +510,10 @@ impl Mempool {
             .chain_err(|| "failed to update mempool from daemon")?;
         let to_remove: HashSet<&Txid> = old_txids.difference(&new_txids).collect();
 
+        // 2. Remove missing transactions. Even if we are unable to download new transactions from
+        // the daemon, we still want to remove the transactions that are no longer in the mempool.
+        mempool.write().unwrap(),remove(to_remove);
+
         // 2. Download the new transactions from the daemon's mempool
         let to_add = Mempool::download_new_mempool_txs(&daemon, &old_txids, &new_txids)?;
 
@@ -518,8 +522,6 @@ impl Mempool {
             let mut mempool = mempool.write().unwrap();
             // Add new transactions
             mempool.add(to_add);
-            // Remove missing transactions
-            mempool.remove(to_remove);
 
             mempool.count
                 .with_label_values(&["txs"])