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>