Offload State Resolution into Rust

This issue has been migrated from [#16502](https://github.com/matrix-org/synapse/issues/16502).

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The aim is to get it off the reactor thread, stop it holding the GIL and ultimately get better performance.

Things to consider:

- [ ] we might need to move `Event` into Rust (Erik has a branch that started this; it's quite old but at the same time it's probably still a good start) #16503
- State res makes DB accesses and those can't be moved into Rust until #16501 is sorted — but there are pieces of code around the DB calls, where we manually yield to the reactor for instance, that are hopefully ripe for moving over