Bug fix: allow querying the "leveldb.num-files-at-level<n>" property

The DB.GetProperty() erroneously used fmt.Scanf() instead of
fmt.Sscanf() when trying to parse the level in properties of the form
"leveldb.num-files-at-level<n>".  This commit changes the method
to use fmt.Sscanf() instead, and adds a test case to check for the
presence of the problem.