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.

also, handle nickname targets in znc.in/playback

Explicit quit and ping timeout behave the same way,
but reattach after abandoning/losing the previous session
(without the break being detected server-side) is more aggressive
about replaying missed messages, at the cost of potential duplication.