#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/datasource/CacheDataSource.java

@@ -143,14 +143,6 @@ public class CacheDataSource implements DataSource {
         return source.getRecordsToPurge(until);
     }
 
-    @Override
-    public void purgeRecords(Set<String> toPurge) {
-        source.purgeRecords(toPurge);
-        for (String name : toPurge) {
-            cachedAuths.invalidate(name);
-        }
-    }
-
     @Override
     public boolean removeAuth(String name) {
         name = name.toLowerCase();
@@ -193,8 +185,8 @@ public class CacheDataSource implements DataSource {
     }
 
     @Override
-    public void purgeBanned(final Set<String> banned) {
-        source.purgeBanned(banned);
+    public void purgeRecords(final Set<String> banned) {
+        source.purgeRecords(banned);
         cachedAuths.invalidateAll(banned);
     }