To facilitate testing the state of cache after the update is preempted
at various points, pass a preemption_source& to update() instead of
calling the reactor directly.
In release builds, the calls to preemption_source methods should compile
to the same direct reactor calls as today. Only in dev mode they should
add an extra branch. (However, the `preemption_source&` argument has
to be shoveled in any mode).
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