Allow situations where the journal contains a big_write(v1) after delete(v2) and v1 < v2

Fixes a crash in the following scenario:
- client issues a delete request (object version is at least 2)
- OSD has time to flush it to the metadata, but doesn't have time to move the journal start pointer on disk
- client overwrites the same object and it gets the version number 1 again
- OSD is restarted and sees delete(v=2), big_write(v=1) in the journal
- dirty_db sequence gets broken and OSD crashes with assert("Writes and deletes shouldn't happen at the same time")

blockstore_write.cpp

@@ -492,7 +492,10 @@ int blockstore_impl_t::dequeue_del(blockstore_op_t *op)
         prepare_journal_sector_write(journal, journal.cur_sector, sqe, cb);
         PRIV(op)->min_flushed_journal_sector = PRIV(op)->max_flushed_journal_sector = 1 + journal.cur_sector;
         PRIV(op)->pending_ops++;
-        // Remember small write as unsynced
+    }
+    else
+    {
+        // Remember delete as unsynced
         unsynced_small_writes.push_back((obj_ver_id){
             .oid = op->oid,
             .version = op->version,