Factor out creation of IMAP command tag

* elmo-imap4.el (elmo-imap4-command-tag): New function. Return IMAP
  command tag.
(elmo-imap4-send-command): Use new function to obtain command tag.
Factor out waiting for server response when parsing

* elmo-imap4.el (with-elmo-imap4-session-process-buffer): New
  macro. Execute BODY in a sessions' process buffer.
(elmo-imap4-command-tag): Use new macro.
(elmo-imap4-session-wait-response-maybe): New function. Wait for
  server response when in parsing state.
(elmo-imap4-send-command): Use new function.