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Each multi-threaded runtime worker prioritizes pulling tasks off of its local queue. Every so often, it checks the injection (global) queue for work submitted there. Previously, "every so often," was a constant "number of tasks polled" value. Tokio sets a default of 61, but allows users to configure this value. If workers are under load with tasks that are slow to poll, the injection queue can be starved. To prevent starvation in this case, this commit implements some basic self-tuning. The multi-threaded scheduler tracks the mean task poll time using an exponentially-weighted moving average. It then uses this value to pick an interval at which to check the injection queue. This commit is a first pass at adding self-tuning to the scheduler. There are other values in the scheduler that could benefit from self-tuning (e.g. the maintenance interval). Additionally, the current-thread scheduler could also benfit from self-tuning. However, we have reached the point where we should start investigating ways to unify logic in both schedulers. Adding self-tuning to the current-thread scheduler will be punted until after this unification.