merge other barrier support

[ehirdoy]

https://github.com/owlbarn/light_actor/blame/master/test/mirage/TODO.md#L18

• lwae: add other Barrier support, @jzstark ?
  • for the barrier support, I've added some initial implementation here: https://github.com/jzstark/light_actor/blob/lda/src/core/actor_barrier_ssp.ml