Give the user more control over lock resolution

[nrc]

Currently, we automatically resolve locks and retry (without bumping the start ts which would be unsound). This is ok as a default, but it would be nice to also allow the user to have more control, i.e., returning an error with the locks and providing a resolve_locks function.

I have started implementing this, but I'm not sure what the API should look like - i.e., how does the user opt-in to auto-retry? There are also lots of details in the retry code to work out.