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)

* Note, this is a re-implementation of
  18c013cd7e511cb83d742e70bb0972e426ae0b0e
  addressing bugs that lead to reverting the patch in
  13ba6527d50f0cd8dae0763e0268ae79f5e5fcf6.

Reported-by: Thorsten Schwander <thorsten.schwander@gmail.com>
Tested-by: Tibor Simko <tibor.simko@cern.ch>