Make _removeThread safe during thread iteration
Make _removeThread safe during thread iteration

Before: `_removeThread` stops a thread and immediately removes it from
the runtime's thread array. If this happens while iterating over the
thread array, such as in the case of clone deletion, some elements of
the array can be missed.

After: `_removeThread` has been renamed to `_stopThread`. It still stops
the thread immediately but it no longer removes the thread from the
runtime's thread array. Instead, `_stopThread` marks the thread for
later removal. Such threads are removed at the top of the next `_step`.

LLK/scratch-vm,LLK/scratch-vm,LLK/scratch-vm