fix #1622

fix #1634 (forward-porting to master)

fix FAIL REGISTER INVALID_USERNAME case

1. Fix auth bypass in the default configuration with the addition of
   server.password (the REGISTER command was allowed before connection
   registration, allowing unauthenticated users to REGISTER and then
   take advantage of skip-server-password)
2. Caution operators against the use of require-sasl without disabling
   user-initiated account registration. (Such a configuration is still valid
   in the case of a public server that requires everyone to register.)

Allow users to set max MySQL connections and connection lifetime;
set a sane default for max connections if it's not present.

fix autoreplay of DMs

I broke this in #1615 / 0d05ab4ff4

fix #1615

fix #1627

implement mass/global messaging

Fix incorrect description of channel ownership in NS INFO

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

fix #1577

Remove debugging loglines for truncation

upgrade dependencies for 2.7 window

small fixes to kick off the 2.7 window

Allow setting the minimum TLS version

Prevent LUSERS stats from getting out of sync when modes are
modified on offline clients.

Allow snomasks to be added via oper config block, even if the oper
doesn't have `ban` or `snomasks` and therefore can't add snomasks
on their own.

Clean up channels during unregistration if necessary.

changelog updates for 2.6.0-rc1

publish the FORWARD 005 token

close remaining 2.6 issues

Warn users that NS UNREGISTER doesn't give them a "do-over";
the account name will remain reserved.

UBAN ADD and DEL need to produce snomasks and loglines

Document permissions structure of CS AMODE

CS INFO with no arguments should list your registered channels