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).

Fixes #1510

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`.

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

Support PROXY protocol v2, including ahead of plaintext connections

thanks @ajaspers!

anything other than a (net.Error) with Temporary() == true
is treated as a fatal error that causes the http server to exit

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

In particular, allow hostnames without periods (like on a LAN).
This shouldn't be a client compability concern since we allow
vhosts without periods.