Let halfops change the channel topic

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.

See #1171.

Add chanmode preventing speech to error message.

Fixes #1182.

1. TAGMSG were incorrectly being considered multilines, because
   Is512() was checking the wrong thing
2. Playback of TAGMSG should depend on event-playback, not on message-tags

This affects /TOPIC, /INVITE, and a few others.

This propagates CS AMODE changes to the actual modes, but not the other
way around.

Also fixes #909.