* Split RegisterAfterUpsert into two
We're going to need to be able to differentiate between Add and Update
for storing events in the _events table.
* Update mocks
This introduces the a `WithTransaction` function, which is then used for all transactional work in Steve.
Because `WithTransaction` takes care of all `Begin`s, `Commit`s and `Rollback`s, it eliminates the problem where forgotten open transactions can block all other operations (with long stalling and `SQLITE_BUSY` errors).
This also:
- merges together the disparate `DBClient` interfaces in one only `db.Client` interface with one unexported non-test implementation. I found this much easier to follow
- refactors the transaction package in order to make it as minimal as possible, and as close to the wrapped `sql.Tx` and `sql.Stmt` functions as possible, in order to reduce cognitive load when working with this part of the codebase
- simplifies tests accordingly
- adds a couple of known files to `.gitignore`
Credits to @tomleb for suggesting the approach: https://github.com/rancher/lasso/pull/121#pullrequestreview-2515872507
