CLIENTS LIST shows information about clients attached to a nick.
CLIENTS LOGOUT allows individual (or all) sessions to be logged out.

SESSIONS is now an alias for CLIENTS LIST.

Fixes #1072.

Link to the new draft PR:
<https://github.com/ircv3/ircv3-specifications/pull/420>

Changes in the spec:

- Use standard replies instead of numerics:
  <https://github.com/ircv3/ircv3-specifications/pull/420/files#diff-70e90beef48dc9cf5d784d1e179ea822R44>
- Allow RENAME to a different case:
  <https://github.com/ircv3/ircv3-specifications/pull/420/files#diff-70e90beef48dc9cf5d784d1e179ea822R42>

This commit makes oragono send the PART-JOIN fallback even on case-only
changes. This is so that clients don't have to worry about oragono's
UTF8 casefolding. See the following comments for further info:
<https://github.com/ircv3/ircv3-specifications/pull/420#issuecomment-668770837>

Misc fixes:

- Remove unused variable,
- Add missing calls to utils.SafeErrorParam,
- Don't fill replies with the user-provided "oldName", for the same
  reason as sending the PART-JOIN fallback.

If force-nick-equals-account is disabled, then this could cause
a client with a non-reserved (or grouped) nick to become always-on.
(This can't happen with `/NS SET always-on true` because we check in
advance.)

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.

now that we have client.destroyed again, we don't need to test the
"0 sessions && brb disabled" condition in both AddSession() and
destroy()

Fixes #480

eliminates Server.configurableStateMutex

Also fix some issues with STATUSMSG