#674 PurgeService: Always register if purging, reduce code duplication

- Rename autoPurging to isPurging: we should always register if a purge task is in progress (regardless if autopurge or not) and deny any new requests
- Reduce the same logic being coded multiple times by calling through the methods
- DataSource: remove purgeBanned in favor of purgeRecords, both do exactly the same thing

src/main/java/fr/xephi/authme/util/BukkitService.java

@@ -135,6 +135,15 @@ public class BukkitService {
         return Bukkit.getBannedPlayers();
     }
 
+    /**
+     * Gets every player that has ever played on this server.
+     *
+     * @return an array containing all previous players
+     */
+    public OfflinePlayer[] getOfflinePlayers() {
+        return Bukkit.getOfflinePlayers();
+    }
+
     /**
      * Safe way to retrieve the list of online players from the server. Depending on the
      * implementation of the server, either an array of {@link Player} instances is being returned,