p2p packet: disconnecting

[moshababo]

Inform a peer that disconnection is imminent, and specify the reason (enum) for that.
If the reason is that the ban threshold exceeded, send him the penalization history as well.
If received, packet payload should be saved to the database.

[kilrau]

Other reasons are stalling (peer became unresponsive, socket closed etc) or shutdown (xucli shutdown).