Make Clear that Parts are Server/Channel/Nick Specific

Multi part messages are of cause specific to a user, channel and server combination. So if you receive something like:

> CRY-1 xyz   #channelA
> CRY-1 xyz   #channelB
> CRY-0 xyz   #channelA
> CRY-0 xyz   #channelB

…the message parts are handled separately even if it was the same user who sent them.
