High memory use: Restrict concurrent processing/terminate activity.

When a node detects high memory use (>80% of quota) stop processing new requests until it falls.  If the system free memory drops below 10%, halt all active requests and release memory in an effort to prevent the node from becoming unresponsive / an OOM kill of the service.

Also provide a feature control flag to enforce a configured node-quota as the hard-limit (i.e. the request-halt threshold) rather than using the system free memory.

In the absence of an explicit node-quota, we'll use 50% of the system memory. (Ideally we want to move away from systems without a node-quota.)