See #1994

See 69448b13a1 / #1969; the compiler can now ensure that a uint64
intended for atomic access is always aligned to a 64-bit boundary.
Convert atomic operations on uint32s and pointers as well.

* Add ACCEPT-tracking functionality (authorizing users to send DMs
  despite +R or other applicable restrictions)
* Sending a DM automatically accepts the recipient
* Add explicit ACCEPT command

KLINE'd clients would produce a QUIT snotice without a corresponding
CONNECT snotice; explicitly suppress the QUIT snotice.

Log it at level INFO for parity with connect / connect-ip

Ensure the pagination window is full by making sure that every history item
gets a replay line in CHATHISTORY output, even TAGMSG.

This reverts commit 5bbee02fe6.

Fix various pagination issues with CHATHISTORY; also undo #491
(msgid munging).

RPL_WHOISACTUALLY should display some arbitrarily chosen IP address and
hostname.

See: https://github.com/ircv3/ircv3-specifications/pull/466

May help clarify TLS configuration issues, possibly others too

It could be the real IP or the proxied IP.

This is a regression introduced in 0d05ab4ff4; playback for an individual
DM target would play all DMs.

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.