Add AvalancheSchedulerSvc and clean up the ForwardSchedulerSvc

This MR factors out all proactive and latency-oblivious task scheduling strategies and mechanisms, which rely on graph-based concurrency control, into a new scheduler - AvalancheSchedulerSvc. The ForwardSchedulerSvc, which had been temporarily hosting these modes as an alternative mode of operation, is cleaned up correspondingly and thus brought back to its primordial form.

For highlights, please see the self-descriptive commit messages.

(At the time of submission, this branch was rebased to the latest upstream master.)