Fix the modal private state thread we use for running expressions on the private state thread (#179799)HEADmain

We have a problem when some code on the private state thread needs to
run an expression. The private state thread is the one that fetches
"raw" events from the Process Plugin and decides how to handle them. But
if the private state thread needs to fetch the processed events to drive
running an expression, it can't also be the thread processing the raw
events.

We solve this by swapping in a modal private state thread just to handle
the events from the expression evaluation. That worked until you could
cause the expression evaluation to happen from Python, because then it
wasn't just the fetching of events that matter, but also the state of
the process and the state of the runlocks. The modal private state
thread is really a modal version of the thread and its associated state.

This patch gathers all the relevant control parameters into a structure
which we can swap in and out when needed.

It also adds a test using the new "was_hit" breakpoint resolver
affordance, which, since it acts as an asynchronous breakpoint callback,
gets run on the private state thread, showing that with the change we
can call expressions in the `was_hit` callback without problems.

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