FileJournal: introduce start_seq header entry

FileStore::header_t::start_seq now encodes the op seq which may be
written at FileStore::header_t::start.  This way, FileStore::open()
can pass a valid sequence number to read_entry for validation.
Otherwise, read_entry has no way of knowing whether a failure of a
read at header.start indicates that the journal was empty, or that
the entry is corrupt.  With start_seq, read_entry can assume
corruption if start_seq <= committed_up_to.

Fixes: #4527
Signed-off-by: Samuel Just <sam.just@inktank.com>