Use enumeration to represent steps instead of strings

[taleldayekh]

Nitpick: Not part of your PR, but I would highly recommend to use a enumeration for such stuff here. Having plain control strings is insecure and instable code. Since this is limited to this component (as it looks on a first gaze to me) I would consider to take the opportunity and change it simply within this PR to make the code better than before. :wink:

Originally posted by @weilbith in https://github.com/raiden-network/light-client/pull/1980#discussion_r461458812