refactor: Clean up the PeerHolder access/iteration/remove

This patch expands the PeerHolder API allowing for encapsulation of
the peer state from the code that needs to iterate over initialized
peers. This cleans up peer iteration/removal allowing for a much
simpler design.

1) Introduce new APIs for PeerHolder:
* initialized_peers_mut()
* initialized_peer_node_ids()
* remove_peer_by_descriptor()

2) Clean up event handler iteration using new APIs
3) Unify the timer_tick and DisconnectPeer event
   disconnect path using new APIs
4) Convert get_peer_node_ids() to use new API

Author: julianknutsen