Add ScopedJobThreadPtr class to be used with JobThread

[alexanderbobrovnik]

b/372515171

[xiaomings]

We are adding a new convention to JobThread, that JoinThread() has to be called prior to destruction.

We could handle this situation better by introducing a new class (e.g. JobThreadHolder) that holds an JobThread object, keeps itself valid before destroying the underlying JobThread. Once the new class is implemented, turn the dtor of JobThread into private and a friend of the new class, so other teammates won't accidentally wrap a JobThread into a std::unique_ptr<>.

We should then revert all previous changes that call ScheduleAndWait() before destroying the JobThread.

[xiaomings]

@borongc is it possible for you to help me review the part in this PR where all changes to Schedule() made during the scoped_ptr<> to unique_ptr<> migration is reverted?