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

lib/explorer.js

@@ -10,11 +10,14 @@ const client = new onode.Client(settings.wallet);
 
 // returns coinbase total sent as current coin supply
 function coinbase_supply(cb) {
-  Address.findOne({a_id: 'coinbase'}, function(err, address) {
+  Address.findOne({a_id: 'coinbase'}).then((address) => {
     if (address)
       return cb(address.sent);
     else
       return cb(0);
+  }).catch((err) => {
+    console.log(err);
+    return cb(0);
   });
 }
 
@@ -810,7 +813,7 @@ module.exports = {
   },
 
   balance_supply: function(cb) {
-    Address.find({}, 'balance').where('balance').gt(0).exec(function(err, docs) {
+    Address.find({}, 'balance').where('balance').gt(0).exec().then((docs) => {
       var count = 0;
 
       module.exports.syncLoop(docs.length, function (loop) {
@@ -821,6 +824,9 @@ module.exports = {
       }, function() {
         return cb(count);
       });
+    }).catch((err) => {
+      console.log(err);
+      return cb(0);
     });
   },