Add call to _maybe_ack() to forward_barrier() to fix #3118.
Under very low application message rates and resilience=on,
the accumulation barrier messages on the other end of the
boundary was caused by not acking here. The _ack_counter
could become an odd number, and _maybe_ack()'s modulo
arithmetic + comparison could never be true, which meant
never sending an ack.
Fixes #3118