thread state coordinator: add exec reset support, remove empty virtual destructors.

Also added a test for the reset handling.  The reset/state clearing happens
as a processed queue event.  The only diff vs. standard processing is that
the exec clears the queue before queueing the activity to clear internal state.
i.e. once we get an exec, we really stop doing any other queue-based activity.


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@218629 91177308-0d34-0410-b5e6-96231b3b80d8