LoadBalancerService does not consider concurrent requests when scheduling

Under concurrent requests to controller, LoadBalancerService will not consider concurrent requests when scheduling to an invoker.

For example, consider `LoadBalacnerServiceObjectTests.scala`, with the following test
```
it should "consider load immediately and jump to next invoker when overloaded" in {
val invokerCount = 10
val hash = 2
val targetInvoker = hash % invokerCount

val invs = invokers(invokerCount).updated(targetInvoker, (InstanceId(targetInvoker), Healthy, 0))
val step = hashInto(LoadBalancerService.pairwiseCoprimeNumbersUntil(invokerCount), hash)

LoadBalancerService.schedule(invs, 1, hash) shouldBe Some(InstanceId((hash + 0) % invs.size))
LoadBalancerService.schedule(invs, 1, hash) shouldBe Some(InstanceId((hash + 1) % invs.size))

}
```

We have:
* set the existing load to 0 to start with
* set the invokerBusyThreshold to 1 on `scheduler()` invocations
* schedule one activation with hash=2
* schedule second activation with hash=2

In this case, but activations are scheduled to InstanceId(2), but the second should be scheduled to next invoker since busy threshold is exceeded.

I believe this is related to the changing of `LoadbalancerData.activationCountPerInvoker()` from sync to async. Since LoadBalancerService is processed with multiple threads, the updates to the LoadBalancerData are not seen immediately by other threads.