When lldb stops to run a breakpoint condition or other callback that has
to happen between the private stop and returning control to the user, it
will run in the state where the public state is still "running". But if
the callback needs to run lldb commands or python code, it needs to see
the correct "stop" state.
We used to handle that by switching the public state to stopped before
running the callbacks. However, that opened a window where we are still
handling the stop event and another thread would be allowed to continue
the target or do other actions that can interfere with that orderly
process.
This patch adds the ability to designate a particular thread as "seeing
the private state" while all other threads see the "public state". Then
when we run a breakpoint callback, no threads but the one that is
actually running the callback will see the state change until the event
has been delivered to the primary state listener.
It also adds a test that while a long-running breakpoint callback runs,
another thread continues to see the state as running.
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