DNM: FileJournal: add committed_up_to to header and check in read_entry

header_t::committed_up_to provides a lower bound for safetly committed
journal entries.  If read_entry fails prior to committed_up_to, we
know we have a corrupt jorunal entry.  This patch also causes
the header to be written on every journal write...should not
go into master.

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