FileJournal,Journal: detect some corrupt journal scenarios

When the checksum or footer are invalid, we will now try to
look at the next entry.  If we find a valid entry, it is likely
that the journal is corrupt.

Signed-off-by: Samuel Just <sam.just@inktank.com>
FileJournal: add testing methods to corrupt entries

Signed-off-by: Samuel Just <sam.just@inktank.com>