RPL_WHOISACTUALLY should display some arbitrarily chosen IP address and
hostname.

Fixes #1733

Configurations with require-sasl and open account registration are valid,
but likely unintended. Show a warning about them.

Remove the CHATHISTORY * and znc.in/playback *self targets,
clean up associated database code, add new mechanisms to play
all missed DMs.

Handle panics in rehash, even if it was triggered by SIGHUP

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

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

Fixes #810

This didn't work correctly for IPv6 or custom nets.
/UNDLINE IP can temporarily be used to reset the throttle.

* Defer account login snomask until after connection registration is complete
* Don't send account-notify for pre-registration clients

Fixes #1407

Support PROXY protocol v2, including ahead of plaintext connections

Add 379 RPL_WHOISMODES

See discussion on #68.