BibDocFile: fix check-md5 with bibdocfsinfo cache

* If the new bibdocfsinfo_cache functionality is enabled by
  setting CFG_BIBDOCFILE_ENABLE_BIBDOCFSINFO_CACHE = 1 in
  invenio(-local).conf, the md5s property of BibDoc objects
  was not correctly initialized and calling `bibdocfile --check-md5`
  would have failed with an unhanded exception.  This is fixed here
  by lazy initialization of the BibDoc.md5s property.  (closes #1249)

Tested-by: Tibor Simko <tibor.simko@cern.ch>