I broke this in #1615 / 0d05ab4ff4

Remove the CHATHISTORY * and znc.in/playback *self targets,
clean up associated database code, add new mechanisms to play
all missed DMs.

Remove debugging loglines for truncation

Implements the new `CHATHISTORY LISTCORRESPONDENTS` API.

AWAY status should be tracked per-session:

1. With auto-away enabled, away status is aggregated across sessions
   (if any session is not away, the client is not away, else use
   the away status that was set most recently)
2. With auto-away disabled, we get the legacy behavior where AWAY
   applies directly to the client

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

Implements #1577, but the issue should remain open until we clean up
the debugging loglines.

1. Text and binary frames are accepted
2. Text frames are sent by default
3. Binary frames are sent to clients who negotiate `binary.ircv3.net`
4. Non-UTF8 data is not accepted (enabling websockets still enables UTFONLY)

Transition most "is an operator" checks to require a specific operator
capability

Track channel join times, use them to optionally enforce history access
restrictions

checkAlwaysOnExpirationNoMutex was respecting registered status, but
always-on clients were not considered registered at the time of the
initial check, so they were being created regardless of expiration.

Add the new utf8-only cap, disallow non-utf8 when websockets are enabled

Fixes #810

INVITE playback (#1409) was buggy, due to the double use of (Item).Params[0]
for the channel name and the recipient nick. Stuff the channel name in
(Item).Message.Message instead.

This didn't work correctly for IPv6 or custom nets.
/UNDLINE IP can temporarily be used to reset the throttle.

Tor listeners should never see an STS cap.

Add an undocumented 'hide-sts' key for listeners that hides the STS cap.
This can be used if the listener is secured at layer 3 or 4 (VPNs,
E2E mixnets). It will be necessary to add the relevant IPs to `secure-nets`.

Store the channel-user modes of always-on clients along with their
channel memberships, restore them on server startup. This will coexist
alongside /CS AMODE, which autoapplies modes to clients on join regardless
of their always-on status.

Record INVITE in DM history for the benefit of offline always-on clients

Clients without event-playback should receive client-only tags in
replayed PRIVMSG / NOTICE.

Instead of building a new serialized message for each recipient,
try to cache them.

See #1171.

This affects RPL_WHOISACTUALLY and possibly others.

CLIENTS LIST shows information about clients attached to a nick.
CLIENTS LOGOUT allows individual (or all) sessions to be logged out.

SESSIONS is now an alias for CLIENTS LIST.

Fixes #1072.

See discussion on #68.