Added garbage collection for disconnected players

- Heartbeat messages are sent out from their own thread to each player
- If a heartbeat message can't be sent to a player, they are removed from the list of players
- Added equals method for players
Tags: #story[1047]

src/main/java/seng302/server/messages/RaceStartStatusMessage.java

@@ -54,6 +54,15 @@ public class RaceStartStatusMessage extends Message {
         writeCRC();
         rewind();
 
-        outputStream.write(getBuffer());
+        if (outputStream == null){
+            return;
+        }
+
+        try{
+            outputStream.write(getBuffer());
+        }
+        catch (IOException e){
+            return;
+        }
     }
 }