Limit BlockPresenceManager map growth be doing the following:

- Use nil map when BlockPresenceManager CID map is empty.
- Delete peer map when from CID map when peer map is empty.
- Remove peers from BlockPresenceManager when peers are pruned from session.

This allows GC to free memory when maps in BlockPresenceManager become empty.

Additional improvement: Do not look for lists of keys or peers in empty maps; return early instead.

Fix for issue #574