* fix #2133

Don't record NICK and QUIT in history for invisible auditorium members

Handling of reserved nicknames is special-cased due to #1594, but we want to send
ERR_NICKNAMEINUSE if the nickname is actually in use, since that doesn't pose any
client compatibility problems.

* implement draft/pre-away
* clean up some subtleties in auto-away aggregation.
* consistently apply auto-away only to always-on
* `AWAY *` should not produce user-visible changes wherever possible

1. Use FAIL instead of WARN
2. Use NICKNAME_RESERVED instead of ACCOUNT_REQUIRED
3. Don't send 433 at all for registered clients, just send the FAIL

Send a WARN NICK ACCOUNT_REQUIRED on attempt to take a reserved nick

Implement the new bot mode spec:
https://github.com/ircv3/ircv3-specifications/pull/439

This avoids conditions where two sessions can be attached to the same client
while not logged into an account.

Fixes #519