how to get the timestamp of hfile, like when it's generated.

[yun peng]

Hi, All
Given ``hfile`` in ``hbase`` is immutable, I want to know the timestamp
like when the ``hfile`` is generated. Does ``hbase`` have API to allow
user-applications to know this? I  need to know in postCompact() stage.

As a first attempt, I have tried using
``StoreFile.getBulkLoadTimestamp()``. By following code snippet, I
constantly get ``NullPointerException``.
  public void postCompact(ObserverContext<RegionCoprocessorEnvironment> c,
Store store, StoreFile resultFile) {
    if(resultFile == null) {
        return;
    }
    int timestampLastCompact = resultFile.getBulkLoadTimestamp();
    ...
  }

Any ideas are appreciated. Thanks.
Yun