In the multi-threaded scheduler, when there are no tasks on the local
queue, a worker will attempt to pull tasks from the injection queue.
Previously, the worker would only attempt to poll one task from the
injection queue then continue trying to find work from other sources.
This can result in the injection queue backing up when there are many
tasks being scheduled from outside of the runtime.

This patch updates the worker to try to poll more than one task from the
injection queue when it has no more local work. Note that we also don't
want a single worker to poll **all** tasks on the injection queue as
that would result in work becoming unbalanced.