Chagne the method getUnderreplicatedFragments to the package private (#…

…4174)

---

###Motivation

Allow to override the getUnderreplicatedFragments to change
the customize the behaviour.

bookkeeper-server/src/main/java/org/apache/bookkeeper/replication/ReplicationWorker.java

@@ -600,7 +600,7 @@ private boolean isLastSegmentOpenAndMissingBookies(LedgerHandle lh) throws BKExc
     /**
      * Gets the under replicated fragments.
      */
-    private Set<LedgerFragment> getUnderreplicatedFragments(LedgerHandle lh, Long ledgerVerificationPercentage)
+    Set<LedgerFragment> getUnderreplicatedFragments(LedgerHandle lh, Long ledgerVerificationPercentage)
             throws InterruptedException {
         //The data loss fragments is first to repair. If a fragment is data_loss and not_adhering_placement
         //at the same time, we only fix data_loss in this time. After fix data_loss, the fragment is still