See #2113 for motivation

Persisting always-on clients was panicking if client X believed it was
a member of channel Y, but channel Y didn't have a record of client X.

This was introduced in 38a6d17ee5

In #2058 we introduced two bugs:

* A nil dereference when an outside user attempts to speak
* Ordinary copy of a modes.ModeSet (which should only be accessed via atomics)

This fixes both issues.

* tweaks to NAMES implementation

* tweak member caching

* add a benchmark for NAMES

See #1994

AMODE +v should allow you to join a +i channel

INVITE did not exempt from +b unless the channel was coincidentally also +i.
This was a regression introduced in v2.4.0.

See #1858: this was the intent all along, but I missed this issue.

The channel mode +R used to both prevent joins by unregistered users,
and prevent unregistered users who happened to be joined from speaking.
This changes the behavior so that +R only prevents joins:

1. This allows users who were invited or SAJOIN'ed to speak
2. To restore the old semantics, chanops can set +RM

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).

Fix auditorium JOIN lines with zero values for the time and msgid tags.

Remove debugging loglines for truncation

Implements the new `CHATHISTORY LISTCORRESPONDENTS` API.

All truncation must be made UTF8-safe.

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

RELAYMSG should respect mutes

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

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

Let halfops change the channel topic

Registered channels should be eagerly created on startup, and should
remain (and be visible in LIST) even when they have no members.

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

Fixes #1260

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.

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

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

Resuming clients without the resume capability would receive
two JOIN lines per channel.