Trigger GC for actors when they tell the cycle detector they're blocked

Prior to this commit, if an actor blocked, it did not run GC to free
any memory it no longer needed. This would result in blocked actors
holding on to (potentially lots of) memory unnecessarily.

This commit causes GC to be triggered when the cycle detector asks
an actor if it is blocked and the actor responds telling the cycle
detector that it is blocked. This should result in memory being
held by blocked actors to be freed more quickly even if the cycle
detector doesn't end up detecting a cycle and reaping the actors.

src/libponyrt/actor/actor.c

@@ -154,6 +154,10 @@ static bool handle_message(pony_ctx_t* ctx, pony_actor_t* actor,
         // We're blocked, send block message.
         set_flag(actor, FLAG_BLOCKED_SENT);
         ponyint_cycle_block(ctx, actor, &actor->gc);
+
+        // trigger a GC if we're blocked and have done work since the last GC
+        if((actor->msgs_since_gc > 0) && (actor->heap.used > 0))
+          pony_triggergc(ctx);
       }
 
       return false;