disconnect command to temporarily disconnect from a peer

[kilrau]

• we already have the regular ban, which still needs Implement behavior for #152 disconnect package "banned" #693 to work reliably
• I hit several occassions where I tried to disconnecting using disconnect ALIAS, so I know we had this before but it is simply very confusing that disconnect is missing from the CLI and I'd like to implement a very simple, temporary disconnect as disconnect node <key/alias> and just print This disconnects you from the peer, but does not prevent the peer re-connecting to you. To permanently disconnect from <alias>, use "ban <alias>".