sql: use a closure to wrap transactions

[moio]

This introduces the a WithTransaction function, which is then used for all transactional work in Steve.

Example:

err := s.WithTransaction(ctx, true, func(tx transaction.Client) error {
		// do things with transaction such as
		_, err := tx.Exec(createTableQuery)
		if err != nil {
			return &db.QueryError{QueryString: createTableQuery, Err: err}
		}
		return nil
	})

Because WithTransaction takes care of all Begins, Commits and Rollbacks, 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

Partly addresses rancher/rancher#48384

Best reviewed commit-by-commit - the vast majority of the changes are actually mechanical. The core of this change is "just" commit 8d7272e

Credits to @tomleb for suggesting the approach.

[ericpromislow]

Just some cosmetic suggestions -- makes sense when I walk through the commits as recommended, and looks good - making this straightforward. And I didn't think we were always rolling back on failures, in some cases we were leaving transactions dangling. Which I guess might work sometimes....

[moio]

And I didn't think we were always rolling back on failures, in some cases we were leaving transactions dangling. Which I guess might work sometimes....

Right, that's the whole point of this PR - we had cases in which dangling transactions blocked new ones. This should really prevent that from happening!

[moio]

Rebased on top of current main and force pushed.

[tomleb]

Looks great, I like how you hid the Commit() / Rollback() functionality to ensure no one is calling it.

[ericpromislow]

LGTM