feat(discord): rewrite + make it thread-safe

The rewrite was necessary to be able to achieve the system to be thread-safe.
It gave the opportunity to organise how the subsystem works.

This also ensures the subsystem is easy to:
- Implement new features!
- Fix bugs without breaking stuff

- Separated the thread into its own class
- Subsystem manages the communication with the thread
- Re-organized the system
- Fix: disable discord-ipc library logger

facades/PC/src/main/resources/logback.xml

@@ -50,4 +50,5 @@
   <logger name="com.snowplowanalytics" level="${logOverrideLevel:-error}" />
   <logger name="io.netty" level="${logOverrideLevel:-warn}" />
 
+  <logger name="com.jagrosh.discordipc" level="OFF"/>
 </configuration>

subsystems/DiscordRPC/src/main/java/org/terasology/engine/subsystem/discordrpc/DiscordRPCBuffer.java

@@ -0,0 +1,71 @@
+// Copyright 2020 The Terasology Foundation
+// SPDX-License-Identifier: Apache-2.0
+
+package org.terasology.engine.subsystem.discordrpc;
+
+import java.time.OffsetDateTime;
+
+/**
+ * A threaded-safe shared buffer used to store information for {@link DiscordRPCThread} to be processed as {@link com.jagrosh.discordipc.entities.RichPresence}
+ *
+ * It helps avoiding allocating unnecessary objects for the rich presence.
+ */
+public final class DiscordRPCBuffer {
+    private String state;
+    private OffsetDateTime startTimestamp;
+    private boolean changed;
+
+    /**
+     * Sets the current party status
+     *
+     * @param state The current party status
+     */
+    public synchronized void setState(String state) {
+        this.state = state;
+        this.changed = true;
+    }
+
+    /**
+     * Returns the current party status
+     *
+     * @return The current party status
+     */
+    public synchronized String getState() {
+        return state;
+    }
+
+    /**
+     * Sets the start of the game
+     *
+     * @param startTimestamp The time when that action has start or null to hide it
+     */
+    public synchronized void setStartTimestamp(OffsetDateTime startTimestamp) {
+        this.startTimestamp = startTimestamp;
+        this.changed = true;
+    }
+
+    /**
+     * Returns the start of the game
+     *
+     * @return The start of the game
+     */
+    public synchronized OffsetDateTime getStartTimestamp() {
+        return startTimestamp;
+    }
+
+    /**
+     * Check if the buffer has changed
+     *
+     * @return if the buffer has changed
+     */
+    public synchronized boolean hasChanged() {
+        return changed;
+    }
+
+    /**
+     * Resets the buffer's change state to false
+     */
+    synchronized void resetState() {
+        this.changed = false;
+    }
+}