Fix mutual close

[marsella]

Closes #337

Before, the merchant's close() function called finalize_mutual_close. Now, the merchant's watcher calls it.

The most substantial change here is an addition of MutualCloseBalances to the merchant database. The merchant watcher cannot tell from the contract itself what the final mutual close balances are, so we need to persist them via the database instead. These changes include

• updating the database schema to include this optional field (sqlx-data.json is the generated file)
• updating the database Rust type to include the field
• adding a getter and setter for the field for a specific channel id

Observation: the getters for each field of the database are a bit clumsy -- they're all identical except for the item being queried. Would it be better to have a single getter for the whole row, and have the caller pull out the field relevant to them?

Testing: I think this scenario is not currently caught by the e2e testing infrastructure. Should determine whether it is and either fix it in this PR or make a new issue + PR to address it.

[marsella]

Testing comments remain important, but we've discussed and it seems like they will be better addressed by a significant update to the testing infrastructure, rather than trying to jerry-rig the script we currently have. See #69 for the plan to address this issue.

Force-push to rebase to main and fix a compile issue with the CI.