Improve Cache class performance

Use hashmap instead of array for caching by discriminator

Optimize `get` method by using `for` loop instead of `forEach`
and returning hashmap cache for queries by discriminator

src/Structures/Server.js

@@ -33,8 +33,6 @@ export default class Server extends Equality {
 		this.afkChannelID = data.afk_channel_id;
 		this.memberMap = {};
 
-		this.members.setHighPerformance();
-
 		var self = this;
 
 		data.roles.forEach((dataRole) => {
@@ -71,8 +69,6 @@ export default class Server extends Equality {
 				}
 			}
 		}
-
-		this.members.setNormalPerformance();
 	}
 
 	detailsOf(user) {