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>