bug#22700: 25.0.91; `erc-echo-timestamps' no longer echoes timestamp for each line

[Noam Postavsky]

> +  (when erc-echo-timestamps
> +    (when-let ((stamp (get-text-property (point) 'timestamp))
> +            (message (format-time-string erc-echo-timestamp-format stamp)))
> +      (unless (string= message (current-message))
> +     (message "%s" message)))))

I'm not an erc user, so perhaps it's just expected to be this way, but
this sounds too aggressive to me.  E.g., if you run a command like
describe-key-briefly its message will immediately be replaced by the
timestamp.

I was thinking erc-echo-timestamp would just save the last timestamp it
sent in a buffer-local variable, and then only send a new message if the
current timestamp is different.