For some reason, various IRC networks do not propagate banlist metadata
such as owner and timestamp when a server rejoins. This causes the
server's banlist entries to contain "set by server" instead of the
original individual who set it and "set on now" instead of when it was
originally set.
This commit modifies PBot's banlist monitoring to only update PBot's
internal banlist metadata when the ban doesn't exist or when the ban is
being updated by an actual individual instead of the server. Now PBot
will remember the original setter and the original timestamp
regardless of whether the IRC server can.
