Another case for FNFE on StoreFileScanner after a flush followed by a compaction

Besides the fix for HStore.getScanners for FNFE on StoreFileScanner after a flush followed by a compaction in HBASE-27484, there is a another case would cause FNFE:

1. When StoreScanner is scanning, there is a flush followed by a compaction. When the flushed is completed in HStore.completeFlush and after it adds the new HStoreFile in StoreEngine, it would notify the StoreScanner.updateReaders by HStore.notifyChangedReadersObservers with the new flushed HStoreFile：

       private boolean completeFlush(List<HStoreFile> sfs, long snapshotId) throws IOException {
       // NOTE:we should keep clearSnapshot method inside the write lock because clearSnapshot may
       // close {@link DefaultMemStore#snapshot}, which may be used by
       // {@link DefaultMemStore#getScanners}.
       storeEngine.addStoreFiles(sfs,
         snapshotId > 0 ? () -> this.memstore.clearSnapshot(snapshotId) : () -> {
         });
       // notify to be called here - only in case of flushes
       notifyChangedReadersObservers(sfs);
       if (LOG.isTraceEnabled()) {
         long totalSize = getTotalSize(sfs);
         String traceMessage = "FLUSH time,count,size,store size,store files ["
           + EnvironmentEdgeManager.currentTime() + "," + sfs.size() + "," + totalSize + ","
           + storeSize + "," + storeEngine.getStoreFileManager().getStorefileCount() + "]";
         LOG.trace(traceMessage);
       }
       return needsCompaction();
     }
   

2. But if before entering StoreScanner.updateReaders , the compaction task starts and it compacts the new flushed HStoreFile and then HStore.closeAndArchiveCompactedFiles is called by CompactedHFilesDischarger to delete the HStoreFile.
3. When StoreScanner.updateReaders continues to execute, it would read already deleted HStoreFile and throws FNFE.