nodes: Use context to perform database queries

Database queries in the SQL elector currently don't use a context. While most are only executed via the monitor function which simply uses a background context which is never cancelled, some of the functions are directly or tranisively called from external contects. As a result, context cancellation would not stop any ongoing database queries.

This MR fixes the issue by passing in contexts to the electors. While at it, it also fixes an inaccurate comment.