[Peers] Tech Debt: disconnect_socket vs. socket_disconnected naming confusion

[julianknutsen]

As a user of LDK, I want a clearer naming convention between PeerManager::socket_disconnected and SocketDescriptor::disconnect_socket so that it is easier to differentiate between the two.

Definition of Done:

• Get 1 more +1 for on_socket_disconnected or the next best thing
• Do the rename

Tech Discussion:
@ariard suggested on_socket_disconnected here.