Adds schedule statistics to memtrace_stream.h.  Implements these
statistics in the streams returned by scheduler_t.  This initial round
includes total switches, time preempts, and direct switch attempts and
successes.

Adds checks that these match the schedule_stats tool's values.

Adds tests of the values to several key scheduler unit tests.

Issue: #6938

…to the separate streams

…heduler-stats

… case.