Adds a new STATUS_IDLE return code, and a corresponding
TRACE_MARKER_TYPE_CORE_IDLE record.

Changes the scheduler behavior to no longer return STATUS_EOF for an
output when the ready queue is empty: instead STATUS_IDLE is returned
until every single input is at EOF.  This results in a more realistic
schedule where other cores can pick up work later rather than
disappearing from the system.

Augments the schedule_stats tool to count idle replies and compute a %
cpu usage metric.  Adds a unit test for counting idles.

Augments the scheduler_launcher to also compute %cpu usage.

Updates all the scheduler tests for the new change.

Adding idle time due to blocking syscalls will be done separately.

Issue: #6471

… analysis tools

…d comments