remove directly accessing nodes.json from queries

[Little-Ben]

Querying FFBot is currently done by two methods:

1. directly accessing community's nodes.json
   This was the first approach and enough for simple queries without the
   need to store further data. This is used for all querries that do not
   deal with "node alarms".

2. periodically import nodes.json to db and then query db
   For the later implemented feature "node alarms" it was necessary to
   store data, therefore I decided to use a database.

In future, all queries should be re-written to use the database instead of
directly accessing nodes.json.