FSHLog may roll a new writer successfully with unflushed entries

Found it when debugging the flakey TestFailedAppendAndSync.

The problem is in waitSafePoint.

      while (true) {
        if (this.safePointAttainedLatch.await(1, TimeUnit.MILLISECONDS)) {
          break;
        }
        if (syncFuture.isThrowable()) {
          throw new FailedSyncBeforeLogCloseException(syncFuture.getThrowable());
        }
      }
      return syncFuture;

If we attach the safe point quick enough then we will bypass the syncFuture.isThrowable check and will not throw FailedSyncBeforeLogCloseException.

This may cause incosistency between memstore and wal.