JournalingObjectStore::journal_replay: don't call committed_thru

It suffices to set last_committed_seq correctly in
FileJournal::open.  This call to committed_thru can race with
the sync thread and trigger the assert(seq >= committed_seq) in
committed_thru.

Backport: emperor (after the sha1 below)
Introduced in d8d27f13e11dcaefd3aa1c049b97c980283da575
Fixes: #6786
Signed-off-by: Samuel Just <sam.just@inktank.com>