model/schedule: revise 'nextThread' data flow

model/schedule: revise 'nextThread' data flow

The ModelChecker::nextThread field was being abused a little in my aging
design. It really should be either a private field (not accessed even via
accessors) or else just a return value / function parameter.

This commit makes a change so that nextThread is a Thread pointer and is
directly supplied to the Scheduler. If it is NULL, then the Scheduler is
allowed to pick its own Thread to run.