kernel: fixup thread monitor locking

The lock in kernel/thread.c was pulling double-duty, protecting
both the thread monitor linked list and also serializing access
to k_thread_suspend/resume functions.

The monitor list now has its own dedicated lock.

The object tracing test has been updated to use k_thread_foreach().

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>