osdc/Journaler: validate header on load and save

osdc/Journaler: validate header on load and save

Previously if the journal header contained invalid
write, expire or trimmed offsets, we would end up
hitting a hard-to-understand assertion much later.

Instead, raise the error right away if the fields
are identifiably bad at load time, and assert that
they're valid before persisting them.

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