Fix possible write stall

The stall occurred during fio Q=128 random write tests with low flusher_count (4).
It was caused by flushers being unable to flush the beginning of the journal
because it contained older writes to an object that also had writes in the very end
of the journal, after dirty_start.

blockstore_journal.cpp

@@ -101,7 +101,7 @@ int blockstore_journal_check_t::check_available(blockstore_op_t *op, int entries
                 : bs->journal.used_start - bs->journal.next_free)
         );
         PRIV(op)->wait_for = WAIT_JOURNAL;
-        bs->flusher->force_start();
+        bs->flusher->request_trim();
         PRIV(op)->wait_detail = bs->journal.used_start;
         return 0;
     }