plugin: Transform posting hooks into transaction hooks.

I feel like posting hooks a case of premature optimization in early
development. This approach reduces the number of special cases in
the code and allows us to more strongly reason about hooks in the
type system.

plugin: Introduce HookRegistry.

This is the layer that keeps track of the different groups of hooks and
can filter them before runtime. The idea here is that you'll be able
to do things like skip hooks that require network access when you don't
have it, or skip CPU-intensive hooks when you don't need them, etc.