Added support for newer mongoose versions

-Mongoose and mongodb dependencies have been updated to the most recent versions in package.json
-Updated the project for use with the newest mongoose major version which required fixing many breaking changes to a number of files in the explorer
-Removed the db.find_address_tx() function as it is unused

NOTE: This update requires mongoose 6.0.10 or higher. If you are getting errors running the explorer after upgrading, it is recommended to run an 'npm update' or manually update mongoose to a more modern version which should fix the problem

bin/instance

@@ -23,9 +23,12 @@ db.connect(dbString, function() {
       server.close(() => {
         var mongoose = require('mongoose');
 
-        mongoose.connection.close(false, () => {
+        mongoose.connection.close(false).then(() => {
           // close the main process now that all http and database connections have closed
           process.exit(0);
+        }).catch((err) => {
+          console.log(err);
+          process.exit(1);
         });
       });
     });