* wl-summary.el (wl-summary-update-folder-info): New function derived from

* wl-summary.el (wl-summary-update-folder-info): New function derived from
`wl-summary-folder-info-update'.
(wl-summary-delete-messages-on-buffer): Use `wl-summary-update-folder-info'
instead of `wl-summary-folder-info-update'.
(wl-summary-sync-update): Use `wl-summary-update-folder-info' instead of
`wl-folder-set-folder-updated'.
(wl-summary-mark-as-read-internal): Use `wl-summary-update-folder-info'
instead of `wl-folder-update-unread'.
(wl-summary-mark-as-answered-internal, wl-summary-prefetch-msg): Pass
the value of `wl-summary-buffer-unread-count' for the second parameter of
`wl-folder-update-unread'.