Journal: commits may not include all journaled seqs

At one point, a commit had to drain the FileStore op
queue.  This is no longer the case.  Consequently, the
journal may have to wait more than one commit for the
filestore to create a stable commit point at a particular
sequence.  Handling this requires two changes:

1) We cannot transition to FULL_WAIT until we receive
a commit_start on a seq >= journaled_seq.
2) We cannot remove the journal completion plug until get
a committed_thru on a seq >= header.start_seq at least as
new as the oldest committed item in the journal.  If on
replay, the journal does not include fs_op_seq, we ignore
it, which is fine since we won't have reported those
entries committed!

Signed-off-by: Samuel Just <sam.just@inktank.com>
FileJournal: fix off by one error in committed_thru

journalq.front().first is the sequence number of the entry
at journalq.front().second.

Signed-off-by: Samuel Just <sam.just@inktank.com>
FileJournal: start_seq is seq+1 if journalq.empty()

This is also the same as journaled_seq + 1 for writeahead
journaling, but not for parallel journaling.

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