* wl-summary.el (wl-summary-sync-force-update): Added argument no-check.

* wl-summary.el (wl-summary-sync-force-update): Added argument no-check.
(wl-summary-sync-update): Ditto.
(wl-summary-goto-folder-subr): Call wl-summary-sync-force-update with
`no-check'.

* wl-message.el (wl-message-buffer-prefetch-timer): New variable.
(wl-message-buffer-prefetch-next): Don't run timer if timer is already running.

* elmo.el (elmo-folder-synchronize): Added optional no-check argument.

* elmo-shimbun.el (elmo-shimbun-folder): Added headers slot.
(elmo-folder-open-internal): Set up headers slot.
(elmo-folder-check): Call elmo-folder-close-internal and
 elmo-folder-open-internal.
(elmo-folder-close-internal): Clean up headers slot.