Pass ctx down to http calls

[avinassh]

Though the ExecContext and QueryContext take ctx parameter, they were not using them. This PR, makes these methods to pass them to callSqld method, which can be used in http calls.

E.g. if a user wants to cancel the operation if it isn't completed in under 1 second, the code would be:

ctx, cancel := context.WithTimeout(context.Background(), 1*time.Second)
defer cancel()
res, err := db.QueryContext(ctx, stmt, args...)