Restore deferred date-stamp insertions in ERC

Restore deferred date-stamp insertions in ERC

* lisp/erc/erc-stamp.el (erc-stamp--recover-on-reconnect): Treat
`erc-stamp--deferred-date-stamp' as a permanent-local variable.
(erc-stamp--date): Document expected possible values for `fn' slot.
(erc-stamp--defer-date-insertion-on-post-modify): Use the function
`ignore' to mean a new `erc-timer-hook' member has been requested.
Use nil to mean one has already run.  Deferred date stamps are new in
ERC 5.6 and Emacs 30.
(erc-stamp--date-mode): Improve doc string.
* test/lisp/erc/erc-scenarios-stamp.el
(erc-scenarios-stamp--date-mode/reconnect): New test.

(cherry picked from commit f6bfa1844b53d6ccd24fd02092ae482d481fc5a5)