i#6831 sched refactor, step 1: Add pImpl idiom (#7074)

This first step toward separating key scheduler code into separate
subclasses by mode of operation adds the pImpl idiom to the scheduler:
the scheduler_t implementation is moved to a private class
scheduler_impl_t, to let us (in a future step) create the appropriate
subclass without changing callers who construct a scheduler_t. This also
helps to split up and modularize the enormous scheduler class.

A future step will move the scheduler_impl_t code into a new file
scheduler_impl.cpp. That was separated out to make the diff easier to
read here.

The bulk of the changes involve:
+ The name change scheduler_tmpl_t to scheduler_impl_tmpl_t.
+ Having to qualify enum types as we do not have C++20's using enum.
+ Having to refer to types in another class now that the impl class is
separate from the outer class.
+ Moving inlined routines out of scheduler.h now that the implementation
is private and scheduler_impl_tmpl_t is an incomplete type in
scheduler.h.
+ Some sched_type_t qualifications are removed due to new (private)
using statements to make the code more readable.

Issue: #6831