ttx service: timeout via context

[adecaro]

Currently, the ttx (token/services/ttx) service is not consistent in the the timeouts what are used. For example, a message is read from a session and a hardcoded timeout is used. The reading should timeout following a context.
The change needs to be applied to all the operation that can timeout in the ttx service.

Wherever possible, we should use the token/services/utils/json/session package to manage sessions.

Here is an example of code that needs to be replace:

msg, err := ReadMessage(session, time.Minute)
if err != nil {
	return errors.Wrap(err, "failed reading signature request")
}

The code should be replaced by using the json-based session wrapper in token/services/utils/json/session.