Add context to all database calls

[SuperSandro2000]

Also fix all unused parameters

I wanted to achieve a maintainable solution that also works for future addition. For that reason I choose to mirror the interface in internal/keppel/database.go and mark all the functions deprecated which then get marked ci golangci-lint.

[majewsky]

I saw several places where WithContext is not used, especially around transactions. The problem is that db.WithContext(ctx) does not have a Begin method. Apparently Gorp expects one to do Begin without context and then use tx.WithContext(ctx) after the transaction has already started.

Since there are some places where WithContext is not used, I tried to have an interface that more strongly discourages misuse. For example, in internal/api/keppel/api.go,

type API struct {
  ...
  db *keppel.DB
  ...
}

could be changed to

type API struct {
  ...
  dbWithoutContext *keppel.DB // do not use directly
  ...
}

func (a*API) db(ctx context.Context) gorp.SqlExecutor {
  return a.db.WithContext(ctx)
}

to make it clear that the DB handle should only be accessed through the helper method that needs a context. But then we have a gorp.SqlExecutor which is missing some vital methods, specifically Begin and Prepare (and therefore also cannot be cast into sqlext.Executor which several library functions take). This looks like a significant issue for context coverage. Opinions?