* elmo2.el (elmo-folder-diff-async-callback): New variable.

* elmo2.el (elmo-folder-diff-async-callback): New variable.
(elmo-folder-diff-async-callback-data): Ditto.
(elmo-folder-diff-async): New function.

* elmo-net.el (elmo-network-session-name-prefix): New variable.
(elmo-network-session-cache-key): Use it as a prefix of the session
name.
(elmo-network-open-session): Ditto.

* elmo-imap4.el (elmo-imap4-status-callback): New local variable.
(elmo-imap4-status-callback-data): Ditto.
(elmo-imap4-server-diff-async-callback): New variable.
(elmo-imap4-server-diff-async-callback-data): Ditto.
(elmo-imap4-local-variables): Added `elmo-imap4-status-callback' and
`elmo-imap4-status-callback-data'.
(elmo-imap4-max-of-folder): Set `elmo-imap4-status-callback' and
`elmo-imap4-status-callback-data' as nil.
(elmo-imap4-server-diff): Ditto.
(elmo-imap4-server-diff-async-callback-1): New function.
(elmo-imap4-server-diff-async): Ditto.
(elmo-imap4-parse-status): Call `elmo-imap4-status-callback'.