Even after the timer fired, we were retaining a pointer to it;
clean those up too along with the other cleanup tasks.

see https://github.com/ircv3/ircv3-specifications/issues/293

during nick change, removeInternal(client) was being called even before checking
whether the new nick was in use or reserved. Reproduction steps:

1. Log in a client 'alice'
2. Log in a client 'bob'
3. bob issues /nick alice, which fails (correctly) with:
:oragono.test 433 bob alice :Nickname is already in use
4. alice issues /msg bob hi, which fails (incorrectly) with:
:oragono.test 401 alice bob :No such nick

(since it's only accessed from the client's own goroutine)

Automatically create the datastore on `oragono run` if it doesn't exist.
See also #302.

1. Remove leaveClientIdle (unused)
2. s/leaveClientActive/leaveClientIdle/
3. make ISON a leaveClientIdle command (some clients send it periodically
   if a /msg window is left open)

add validation for isupport tokens

According to jwheare, ISO 8601 doesn't specify the behavior one way
or the other, and the server-time spec is also ambiguous, but this
is safest.

Fix spurious bidi rule violations in casefolding channel names
by stripping the # before starting the casefolding.

* fix a race condition: a call to `Write` does not spawn a writer goroutine
  if the trylock is held, so `BlockingWrite` must check for fresh data after
  releasing the trylock
* streamline some close/finalize logic

1. Handle PROXY lines with IPv6 addresses starting with ::
(similar to WEBIRC in issue #211)

2. Strip v6 mapping from v4 addresses when handling proxied IPs.

According to https://defs.ircdocs.horse/defs/numerics.html, 330 RPL_WHOISACCOUNT
takes 4 parameters: `<client> <nick> <authname> :<info>`. We were omitting
the second parameter (the target nick).

Also refactor locking.