The inbound parser ignored METADATA entirely, so on a services netburst
(after a restart/relink) the ircd's replayed 'accountname' metadata was
dropped — every logged-in user looked logged out to services until they
re-identified, losing ChanServ access. Parse METADATA <uid> accountname
into a new AccountLogin event that restores account_of (empty = logout);
our own echoes and other keys are filtered.