mds: Introduce mds_journal_format config setting

This is useful for testing, so that we can create an
old-style journal and then test the version migration
by changing the config setting.

Signed-off-by: John Spray <john.spray@inktank.com>
osdc/Journaler: include stream_format in dump()

Signed-off-by: John Spray <john.spray@inktank.com>
Fix JournalStream::read length assertion

This was checking for new-format length even
when reading from an old-format journal.

Signed-off-by: John Spray <john.spray@inktank.com>