This adds a new field to upstreams, members, which is a casemapped map
of upstream users known to the soju. The upstream users known to soju
are: self, any monitored user, and any user with whom we share a
channel.
The information stored for each upstream user corresponds to the info
that can be returned by a WHO/WHOX command.
We build the upstream user information both incrementally, capturing
information contained in JOIN and AWAY messages; and with the bulk user
information contained in WHO replies we receive.
This lets us build a user cache that can then be used to return
synthetic WHO responses to later WHO requests by downstreams.
This is useful because some networks (eg Libera) heavily throttle WHO
commands, and without this cache, any downstream connecting would send 1
WHO command per channel, so possibly more than a dozen WHO commands,
which soju then forwarded to the upstream as WHO commands.
With this cache most WHO commands can be cached and avoid sending
WHO commands to the upstream.
In order to cache the "flags" field, we synthetize the field from user
info we get from incremental messages: away status (H/G) and bot status
(B). This could result in incorrect values for proprietary user fields.
Support for the server-operator status (*) is also not supported.
Of note is that it is difficult to obtain a user "connected server"
field incrementally, so clients that want to maximize their WHO cache
hit ratio can use WHOX to only request fields they need, and in
particular not include the server field flag.
Co-authored-by: delthas <delthas@dille.cc>
