Fix broken main branch caused by wrong getBitSet method

### Motivation

Currently the main branch is broken by the concurrent merge of
apache#153 and
apache#151.

### Modifications

Add a dummy `getBitSet` implementation to `BatchMessageAcker` and the
correct implementation for `BatchMessageAckerImpl`.

lib/BatchMessageAcker.h

@@ -43,7 +43,10 @@ class BatchMessageAcker {
         return prevBatchCumulativelyAcked_.compare_exchange_strong(expectedValue, true);
     }
 
-    const BitSet& getBitSet() const noexcept { return bitSet_; }
+    virtual const BitSet& getBitSet() const noexcept {
+        static BitSet emptyBitSet;
+        return emptyBitSet;
+    }
 
    private:
     // When a batched message is acknowledged cumulatively, the previous message id will be acknowledged
@@ -80,6 +83,8 @@ class BatchMessageAckerImpl : public BatchMessageAcker {
         return bitSet_.isEmpty();
     }
 
+    const BitSet& getBitSet() const noexcept override { return bitSet_; }
+
    private:
     BitSet bitSet_;
     mutable std::mutex mutex_;