Close DB connection after notify.
Manage DB connections in /sse.

* Add 2nd DB that only handles listen/notify connections.
* Close auth query DB connection immediately. Connection used
  for query for login_required check was remaining open
  until request completed. By closing it manually, we
  only have 1 long connection to DB, rather than 2.
* Close DB connection after 3 SSE messages to make
  sure connections don't pile up when clients disconnect.