open_doc_with_docinfo failure due to an invalid docinfo being passed

Thie issue described below occurs very rarely during mixed I/O tests on Windows (concurrent GET/SET). The root cause may be the same as MB-6199, however, EP Engine should do better in terms of logging this rare error case for future triaging.

EP Engine's batch reader (bgfetcher) runs into a failure (invalid docinfo, no doc can be found by the docinfo) from couchstore_open_doc_with_docinfo.