[FIX] http: Consume less cursors for session checks

- Each time we check if a session is valid we create a new cursor.
  This could lead to issues with db_maxconn that limits the number of
connections to the postgresql server.
  In a perfect world, a worker should use a single connection to
postgres to process the request.
  The only known side effect is that the cursor is created earlier in
the execution of the code.

- This commit fixes issues with the longpolling raising
Psycopg2.PoolError exceptions on databases with a lot of clients.