Improve whitelist feature

* Use a primary autoincrementing primary key because primary keys need
  to be unique
* Cache whitelist on bot startup / only read from database once
* "Externalize" whitelist checking to the messageCreate function.

server/createWhitelistTable.js

@@ -4,7 +4,8 @@ module.exports = async function () {
     return new Promise ((resolve, reject)=>{
         db.run(
             `CREATE TABLE IF NOT EXISTS whitelist (
-                        serverId TEXT PRIMARY KEY, 
+                        Id INTEGER PRIMARY KEY AUTOINCREMENT, 
+                        serverId TEXT, 
                         command TEXT,
                         dateAdded INTEGER)`,
             (err) => {