Expose method to fetch internal state on an OffchainStateInstance

[hattyhattington17]

Created to address 1935.

Exposes a method on OffchainStateInstance fetchInternalState which fetches settled actions from the archive node and rebuilds the internal state trees.

[45930]

This looks like the right change. Did you test with any examples?

[hattyhattington17]

I was actually pretty unsure of how to unit test this! First I tried creating separate instances of OffchainStateInstance and generating a settlement proof on one, but then the other was also updated because internally they're the same instance (both memoized by address). I was thinking of creating a totally separate OffchainState and smart contract and then instantiating them at the same address as the existing one would create two actually separate OffchainStateInstances, but then I also thought that it might be kind of a heavy test for a small method. Think it's worth adding?

[45930]

I think it would be easiest to test on lightnet or devnet so you can have 2 different nodeJS threads running different instances of offchain state and talk to the same blockchain... that ought to work. You could add it to the integration tests, or just run a test yourself to make sure this let's you sync back up with the chain after the chain diverges from your local state.