Fix collections, TxPool indexes

Author: emiliorizzo

src/lib/collections.js

@@ -200,15 +200,18 @@ export default {
         },
         unique: true
       }
-    ],
-    TxPool: [
+    ]
+  },
+  TxPool: {
+    indexes: [
       {
         key: {
           timestamp: -1
         }
       }
     ]
   },
+
   PendingTxs: {
     indexes: [
       {