Make Clear that Parts are Server/Channel/Nick Specific

[lkiesow]

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.