@Override
        public Long answer(InvocationOnMock invocation) throws Throwable {
          WALKeyImpl key = invocation.getArgument(1);
          MultiVersionConcurrencyControl.WriteEntry we = key.getMvcc().begin();
          key.setWriteEntry(we);
          return 1L;
        }
Utility method to setup a WAL mock.
<p/>
Needs to do the bit where we close latch on the WALKeyImpl on append else test hangs.
@return a mock WAL
hbase-server/src/test/java/org/apache/hadoop/hbase/regionserver/TestHRegion.java