agenda: Fix prompt buildup in agenda.

Every time a delimited continuation was resumed, we were re-parameterizing.

* 8sync/agenda.scm (current-agenda-prompt): Remove.
(8sync-abort-to-prompt): Use (%current-agenda-prompt) parameter.
(%current-agenda-prompt): New prompt, deprecating %current-agenda.  This
is a slight optimization; we were only using the agenda for the prompt,
so we can remove the indirection of looking up the agenda prompt tag
every time.
(%current-agenda): Removed, see above.
(start-agenda): Move parameterization of %current-agenda-prompt,
current-read-waiter, and current-write-waiter here.
(agenda-run-once!): Remove parameterization.
Also remove comment which is irrelevant now that we're no longer using
an immutable-record-type on the agenda.