irclib: Store authentication status of other users

[progval]

trafficstars

using a combination of:

• https://ircv3.net/specs/extensions/extended-join.html
• https://ircv3.net/specs/extensions/account-notify
• listening for PART/KICK/QUIT to trim the list

But it should not be enabled if the two caps above are not available, as they don't provide tracking over time.

Also, it only works for users we share a channel with, unless we MONITOR them and https://ircv3.net/specs/extensions/extended-monitor.html is enabled

[koobs]

Thanks @progval.

For other readers, this request issue is an outcome of IRC conversation on the following use case:

• Ability to track channel stats (via ChannelStats, for example)
• Only for registered/identified users (a strong unique key: 'account')