Use PFS as backend

[manuelwedler]

As @palango pointed out here #74 (comment), it might make sense to get rid of the current Explorer backend and add the needed APIs to the PFS as it has all the data needed.

We should check first if there is really all the needed data in the PFS and if it can be easily gathered in the API.

It might also be a good time to split the API calls for every token network, because the current API is causing performance issues (see #181).