Refactor PacketTunnelActor's state logic to use a Reducer

[acb-mv]

This is an experimental refactoring of the PacketTunnelActor state machine, which splits the state processing logic into two stages: a stateless function (a Reducer) which takes incoming events (Commands) and existing state and produces updated state and zero or more Effect values, and stateful code within the actor that executes the Effect values, producing effects. This is an instance of the Reducer architecture, and has the effect of making the state transition logic purely stateless and allowing it to be tested and reasoned about without concern for side effects.

This branch is not on main but on features/ios-post-quantum, as to incorporate extensions to the state flow used in the PQ work. Despite this, it is not part of the PQ work as such.

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This change is 

[rablador]

Reviewed 6 of 6 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved