Support dynamic concurrency control

[sebastianburckhardt]

Looks like the Locavore load balancing is not working on the latest bits.
Specifically, this test:

curl --max-time 6000 https://functionssb3.azurewebsites.net/CollisionSearch/flat-parallel -d 1000

is not showing any signs of load rebalancing, all activities are executed locally.

[sebastianburckhardt]

Looks like the problem disappears when setting maxConcurrentActivityFunctions = 10. So the problem seems to be that CPU overload (100 cpu-intensive tasks running at same time) wreaks havoc on the lease management, causing terrible churn. Since I am not seeing the same issue for AzureStorage backend, I guess what is happening is that the latter "self-regulates": if the system is overloaded, it cannot fetch more activities to execute. Here however, the activities are all local and ready to go.

I believe we need some regulation mechanism so that if the node is already struggling with CPU load, it does not start new activities, but lets them sit in the queue from where they can be redistributed.