Adds the server-side watch subscription a buddy list needs: WatchUser (5)
asks the server to keep pushing a user's status, UnwatchUser (6) cancels it.
The reply to a watch carries the user's current status and share statistics,
which merge into the existing per-user snapshot read via Client::user_info.
Issue #47 asked for AddUserList (141) / WatchedUserAdded (142) /
WatchedUserRemoved (143). Those codes are EnableRoomInvitations,
ChangePassword and AddRoomOperator in the Soulseek protocol -- there are no
such messages on the wire. Watching is codes 5 and 6, so that is what this
implements.
- message/server/watch_user.rs: WatchUserHandler (code 5), tolerant of an
absent user and of a truncated frame
- MessageFactory::build_watch_user / build_unwatch_user
- Client::watch_user / unwatch_user / watched_users
- The watch reply has no privileged flag, so it preserves whatever an
earlier GetUserStatus recorded rather than asserting false
