Fix thread synchronization issue in the fixed-lag smoother

[svwilliams]

The started/ignited variables are accessed from multiple threads. Changing them to atomic instead.

[efernandez]

I thought the pending_transactions_mutex_ was protecting the started_ and ignited_ flags, but I see some methods where they are changed without locking that mutex:

• write access:

  • fuse/fuse_optimizers/src/fixed_lag_smoother.cpp

    Lines 381 to 382 in 3b59771

    	started_ = false;
    	ignited_ = false;

  • fuse/fuse_optimizers/src/fixed_lag_smoother.cpp

    Line 130 in 3b59771

    started_ = true;

• read access:

  • fuse/fuse_optimizers/src/fixed_lag_smoother.cpp

    Line 414 in 3b59771

    if (started_ && transaction_time < start_time_)

  • fuse/fuse_optimizers/src/fixed_lag_smoother.cpp

    Line 230 in 3b59771

    if (!started_)

  • fuse/fuse_optimizers/src/fixed_lag_smoother.cpp

    Line 478 in 3b59771

    status.add("Started", started_);

So this LGTM.