final changelog and documentation updates

Fix Docker

fix #848

fix 3 separate bugs contributing to #812

unregistering an always-on client would produce
"attempting to persist logged-out client : x"
because the client was always-on, but also being ejected

It was only initialized for always-on clients, not for regular clients.
This explains a lot in terms of #812 failing to reproduce.

branding, manual, and changelog updates for 2.0

fix #838

fix #836

fix #833

fix #831

fix #829

also, handle nickname targets in znc.in/playback

change "last signoff" tracking to "last seen"

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.

Make docker build with go 1.14

partial fix for #812

fix #821, maybe