Use a lock file (sbbslist.json.lock) to protect the database (sbbslist.json)

from corruption during concurrent operations. Although the list file is not
open shareable, I was observing corruption on Vertrauen when multiple sbbslist
tasks (e.g. verification and maintenance) were running concurrently on
different systems. The .lock file method is crude, but is working to protect
the list from concurrent opens of any kind (read or write).